GLP-1 Weight Loss Medication Glossary: 80+ Terms Defined

A

Active pharmaceutical ingredient (API)

The biologically active substance in a medication. In compounded GLP-1 medications, the API (semaglutide or tirzepatide) comes from a bulk drug substance supplier and is compounded by a state-licensed 503A pharmacy or a 503B outsourcing facility. The API is the chemical that produces the therapeutic effect, but it is not the complete medication. Formulation, purity, potency, and sterility also matter.

Adiposity

Excess body fat. Clinical assessment goes beyond BMI to consider fat distribution, particularly visceral adiposity (fat around internal organs). Adiposity is one measure of whether weight loss translates to improvements in metabolic health.

Adverse event

An unwanted medical occurrence that happens during GLP-1 treatment. Common adverse events include nausea, vomiting, constipation, and injection site reactions. Adverse events are different from contraindications: they may occur during treatment, but contraindications mean you should not start treatment at all.

Agonist

A medication that binds to a receptor and activates it, mimicking the effect of a natural hormone. GLP-1 medications are receptor agonists: they activate the GLP-1 receptor, which triggers the effects associated with the natural GLP-1 hormone.

B

Basal metabolic rate (BMR)

The number of calories your body burns at rest to maintain basic functions like breathing and circulation. BMR declines with age and is higher in people with more muscle mass. It is different from “metabolism” in casual conversation, which usually refers to overall energy expenditure.

Black box warning

The strongest safety warning the FDA places on prescription medications, appearing in a black-bordered box at the top of prescribing information. FDA-approved semaglutide and tirzepatide carry a boxed warning for thyroid C-cell tumors (based on studies in rodents; it is unknown whether they cause these tumors in humans)^[3][7]. This warning means there is a serious potential risk, but it does not mean the medication is prohibited. It means the risk should be discussed with your prescriber and weighed against benefits. See the detailed GLP-1 and thyroid cancer guide at /patient-guide/side-effects/glp-1-thyroid-cancer-warning/.

BMI (Body Mass Index)

A screening measure calculated from height and weight (kg/m2). FDA-approved semaglutide and tirzepatide for chronic weight management are labeled for adults with a BMI of 30 or higher, or 27 or higher with at least one weight-related condition^[6][7]. A licensed provider decides whether treatment is appropriate. Note that BMI does not distinguish fat from muscle, so it has important limitations. A very muscular person might have a high BMI without excess body fat, for example.

Body composition

The proportion of your body weight that is fat versus lean mass (muscle, bone, organs, water). Two people at the same weight can have very different body compositions and very different health profiles. GLP-1 medications affect body composition differently than diet alone: approximately 25-30% of weight lost on GLP-1 medications is lean mass^[2].

C

Certificate of Analysis (COA)

Lab documentation confirming a specific batch of compounded medication meets potency and purity specifications. Usually issued by the supplier or a testing laboratory. A quality indicator for compounded medications. You can ask a compounding pharmacy whether it can share a COA or batch testing results for your medication. See guidelines for choosing a safe compounding pharmacy at /cost/compounded-medications/choose-safe-compounding-pharmacy/.

Comorbidity

A health condition that exists alongside another. Weight-related comorbidities include hypertension, type 2 diabetes, high cholesterol, sleep apnea, cardiovascular disease, and others. The FDA-approved weight management labels include adults with a BMI of 27 or higher who have at least one weight-related condition^[6][7]. A licensed provider decides whether treatment is appropriate.

Compounded medication

A medication prepared by a state-licensed compounding pharmacy for an individual patient’s prescription (503A), or by an FDA-registered outsourcing facility (503B). Compounded GLP-1 medications are not FDA-approved products. The FDA does not review them for safety, effectiveness, or quality, and they are not the same as brand-name products; they may differ in formulation, purity, and potency. Compounding is legal only under the conditions set by federal and state law, and a prescription from a licensed provider is required. Compounding is used when a patient needs a customized dose, formulation, or strength that is not available commercially. See the comparison of compounding types at /cost/compounded-medications/503a-vs-503b-pharmacy/.

Contraindication

A condition or factor that makes a particular treatment inadvisable. The labeled contraindications for FDA-approved semaglutide and tirzepatide are a personal or family history of medullary thyroid carcinoma (MTC), MEN 2, and a prior serious allergic (hypersensitivity) reaction to the medication or its ingredients^[6][7]. Other conditions are warnings rather than contraindications: the labels say these medications are not recommended in severe gastroparesis and have not been studied in people with a history of pancreatitis. A licensed provider weighs these factors. See the full contraindications list at /weight-loss/faq/glp-1-contraindications/.

C-cell

A neuroendocrine cell in the thyroid that produces calcitonin (a hormone that regulates calcium). Cancer that starts in C-cells is called medullary thyroid carcinoma (MTC). Animal studies showed that GLP-1 receptor agonists can cause C-cell tumors in rodents. It is unknown whether they cause these tumors in humans, which is why semaglutide and tirzepatide labels carry a boxed warning.

CGMP (Current Good Manufacturing Practice)

Federal standards for the quality and safety of drug manufacturing. 503B outsourcing facilities are registered with the FDA and are required to follow CGMP standards. 503A compounding pharmacies follow state pharmacy regulations, which do not require CGMP certification.

D

Dose escalation

The gradual increase of medication dose over time. On the FDA-approved weekly injection labels for weight management, semaglutide starts at 0.25 mg and increases every 4 weeks (0.5 mg, 1 mg, 1.7 mg, then 2.4 mg, the usual maintenance dose; the label allows up to 7.2 mg), and tirzepatide starts at 2.5 mg and increases in 2.5 mg steps after at least 4 weeks (maintenance doses of 5 mg, 10 mg, or 15 mg)^[6][7]. The gradual increase is meant to reduce GI side effects and allow the body to adjust. Compounded products are not covered by these labels, and schedules vary by prescriber. Slower dose escalation may be used if you experience significant side effects.

Diabetic ketoacidosis (DKA)

A serious complication of diabetes where the body produces too many ketones and blood becomes too acidic. GLP-1 medications are not a substitute for insulin, and DKA has been reported when insulin doses were reduced too quickly. The FDA-approved semaglutide and tirzepatide products discussed on this site are not approved for type 1 diabetes, and their use in type 1 diabetes has not been established.

Dyslipidemia

Abnormal levels of fats (lipids) in the blood, including high LDL cholesterol, low HDL cholesterol, or high triglycerides. Often found alongside insulin resistance and obesity. GLP-1 medications may improve dyslipidemia through weight loss and metabolic effects.

E

Efficacy

How well a medication works under controlled trial conditions. Different from effectiveness (how well it works in real-world practice).

Endocrinologist

A medical provider who specializes in hormones and endocrine diseases. The Endocrine Society has issued clinical guidance on GLP-1 use for weight management and diabetes. Some prescribers refer complex cases to an endocrinologist.

F

Food noise

Informal term for constant, intrusive thoughts about food, cravings, and eating. Many people taking GLP-1 medications report less food noise, likely because these medications act on appetite centers in the brain.

Fungal contamination

Contamination of a compounded medication with fungal organisms during preparation. A serious quality failure that makes a compounded medication unsafe. Sterile compounding standards (USP 797) require controls designed to prevent contamination. You can ask a pharmacy how it tests batches for sterility and potency.

G

Gastric emptying

The rate at which food leaves your stomach and enters your small intestine. GLP-1 medications slow gastric emptying, which means food stays in your stomach longer, making you feel full longer. This contributes to appetite suppression. Because GLP-1 medications further slow emptying, the FDA-approved semaglutide and tirzepatide labels say they are not recommended in people with severe gastroparesis (excessively slow gastric emptying)^[6][7].

GIP (Glucose-dependent insulinotropic polypeptide)

A gut hormone that stimulates insulin release in response to glucose. Tirzepatide activates both GLP-1 and GIP receptors, which is why it is called a dual agonist. In the SURMOUNT-5 trial, adults with obesity taking tirzepatide lost more weight on average over 72 weeks than those taking semaglutide^[8]. Results vary. Trial results describe study participants and are not a prediction for any individual.

GLP-1 (Glucagon-like peptide-1)

A gut hormone released after eating that stimulates insulin secretion, reduces glucagon (a hormone that raises blood sugar), slows gastric emptying, and suppresses appetite. GLP-1 receptor agonist medications mimic this hormone at higher levels to produce these effects. GLP-1 was discovered in 1987 and has been used to treat type 2 diabetes since 2005^[4].

Glucagon

A hormone that raises blood sugar by stimulating the liver to release glucose. Released when blood sugar drops or during stress. GLP-1 suppresses glucagon, which helps prevent blood sugar spikes.

Gluconeogenesis

The process by which the liver manufactures new glucose from non-carbohydrate sources (like amino acids and glycerol). Occurs when blood sugar is low or during fasting. Insulin (and GLP-1, which stimulates insulin) suppresses gluconeogenesis.

Gastroparesis

A condition where the stomach empties too slowly. GLP-1 medications further slow gastric emptying, and the FDA-approved semaglutide and tirzepatide labels say they are not recommended in severe gastroparesis. Symptoms include nausea, vomiting, bloating, and early satiety.

H

HbA1c (Hemoglobin A1c)

A blood test measuring average blood sugar over 2-3 months. Used to diagnose and monitor diabetes^[5]. Normal: less than 5.7%. Pre-diabetes: 5.7-6.4%. Diabetes: 6.5% and above. Abbreviated as “A1c” or “HbA1c.”

Healthcare provider

A licensed medical professional authorized to diagnose and prescribe medications. At most telehealth programs, a licensed provider (a physician, nurse practitioner, or physician assistant) reviews your health history and decides whether a GLP-1 prescription is medically appropriate.

Homeostasis

The body’s process of maintaining stable internal conditions (blood sugar, pH, temperature, hormone levels). The body has many homeostatic mechanisms designed to restore you to a “set point.” This is why weight loss is so difficult: your body actively resists falling below its set point.

Hypertension

High blood pressure. A common weight-related comorbidity. GLP-1 medications may improve hypertension through weight loss and metabolic effects, though blood pressure lowering is not a primary mechanism of GLP-1 action.

Hypothalamus

A region of the brain that regulates hunger, satiety, temperature, and hormone release. The hypothalamus contains GLP-1 receptors that influence appetite. GLP-1 medications activate hypothalamic receptors to reduce appetite signaling.

I

Incretin

A class of gut hormones (including GLP-1 and GIP) that stimulate insulin release in response to food. GLP-1 medications are sometimes called incretin mimetics because they mimic incretin hormone action.

Insulin resistance

A condition where cells respond less effectively to insulin, requiring the pancreas to produce more. A core feature of metabolic syndrome and type 2 diabetes. Insulin resistance worsens with age, weight gain, and sedentary lifestyle. GLP-1 medications improve insulin sensitivity over time, partly through weight loss and partly through direct metabolic effects.

Inert ingredient

A component of a compounded medication that is not the active pharmaceutical ingredient. Examples include preservatives, buffers, stabilizers, and sterile water for injection. The inert ingredients must also meet USP (United States Pharmacopeia) quality standards and be appropriate for injection or oral use.

Injection site reaction

Local inflammation, redness, itching, or pain at the site where you inject a medication. Common with injectable GLP-1 medications but usually mild and temporary. Rotating injection sites (abdomen, thigh, upper arm) can reduce the frequency of reactions.

L

Lean mass

Body tissue that is not fat, including muscle, bone, organs, and water. Approximately 25-30% of weight lost on GLP-1 medications is lean mass. This is why preserving lean mass through strength training and adequate protein during treatment is important.

LegitScript

A private company that certifies online pharmacies and telehealth companies that apply and pass its review. Checking whether a company is LegitScript certified is one check you can run yourself. Certification reflects LegitScript’s review at a point in time; it is not a government license and does not guarantee the quality of a specific medication.

Lipolysis

The breakdown of fat stored in adipose tissue into free fatty acids that can be used for energy. Weight loss on GLP-1 medications involves lipolysis as stored fat is used for energy.

M

MACE (Major Adverse Cardiovascular Events)

A composite cardiovascular outcome measure used in clinical trials, typically including heart attack, stroke, and cardiovascular death. Some GLP-1 medications have reduced MACE in large clinical trials. For example, in the SELECT trial, semaglutide 2.4 mg reduced MACE by 20% compared with placebo in adults with existing cardiovascular disease and overweight or obesity without diabetes^[9]. Not every GLP-1 medication has shown this benefit.

Medullary thyroid carcinoma (MTC)

A rare type of thyroid cancer affecting C-cells that produce calcitonin^[3]. Personal or family history of MTC is an absolute contraindication to GLP-1 medications. See detailed information at /patient-guide/side-effects/glp-1-thyroid-cancer-warning/.

MEN 2 (Multiple Endocrine Neoplasia syndrome type 2)

A rare inherited syndrome that causes tumors of certain endocrine glands, including medullary thyroid carcinoma. An absolute contraindication to GLP-1 medications. If you have a family history of MEN 2 or thyroid cancer, tell the prescriber before starting GLP-1 treatment.

Metabolic rate

The rate at which your body burns calories at rest. Metabolic rate declines with age and increases with muscle mass. During severe calorie restriction, metabolic rate also declines (adaptive thermogenesis), which is why sustained weight loss without medication becomes progressively harder.

Metabolic syndrome

A cluster of five conditions (elevated blood pressure, high blood sugar, excess waist circumference, high triglycerides, low HDL cholesterol) that together increase cardiovascular and metabolic risk. GLP-1 medications may improve all five parameters. See more at /patient-guide/specific-conditions/glp-1-metabolic-syndrome/.

Medication tolerance

A condition where the body becomes less responsive to a medication over time, requiring higher doses to produce the same effect. Some patients report that GLP-1 effects diminish over months or years. Tolerance is different from a lack of effect: it occurs after initial efficacy.

N

Nausea

A common side effect of GLP-1 medications, especially during dose escalation. Nausea is temporary for most patients and improves within days or weeks as the body adjusts. Slowing dose escalation, taking medication with food (if prescribed), or adjusting meal composition can help manage nausea.

Neuropeptide Y (NPY)

A brain neurotransmitter that increases appetite. GLP-1 acts partly by suppressing NPY signaling, which reduces food-seeking behavior.

Brand-Name Manufacturer (Semaglutide)

The pharmaceutical company that manufactures branded semaglutide products does not manufacture or endorse compounded GLP-1 medications.

O

Obesity

A medical condition defined as BMI 30 or higher^[1]. Obesity is a complex disease with genetic, metabolic, environmental, and behavioral components, not a failure of willpower. FDA-approved GLP-1 medications for chronic weight management are labeled for adults with a BMI of 30 or higher, or 27 or higher with at least one weight-related condition^[6][7].

Off-label use

Using a medication for a purpose not listed in its FDA-approved prescribing information. Providers can prescribe medications off-label for clinical reasons. Many GLP-1 applications (for example, PCOS, certain neurological conditions, or weight loss at BMI 27-30 without comorbidities) are off-label, meaning the FDA-approved product is not labeled for that use. Compounded products have no FDA-approved labeling.

P

PCAB (Pharmacy Compounding Accreditation Board)

A voluntary accreditation program for compounding pharmacies (now run by ACHC), reviewing standards above the regulatory minimum. PCAB-accredited pharmacies have passed a survey covering quality assurance, sterile compounding, and staff training. PCAB accreditation is one thing you can check when looking at a pharmacy. See guidance on selecting a safe pharmacy at /cost/compounded-medications/choose-safe-compounding-pharmacy/.

Pancreatitis

Inflammation of the pancreas. Acute pancreatitis has been reported with GLP-1 use, and the labels warn about it. These medications have not been studied in people with a history of pancreatitis, so tell the prescriber if you have had it.

Peptide

A short chain of amino acids. Semaglutide and tirzepatide are peptides, which is why they must be injected (they would be broken down in the digestive system if swallowed). Semaglutide is also available as an oral tablet formulation, which uses special technology to protect it from digestion.

USP-grade

A term used to describe ingredients that meet the purity and quality standards set by the US Pharmacopeia (USP). USP is a nonprofit scientific organization that sets quality standards for medicines; it is not an FDA designation and does not imply FDA review of a finished compounded product. Licensed compounding pharmacies are expected to source their active ingredients from suppliers that meet USP standards.

Pharmacokinetics

How the body processes a medication over time, including absorption, distribution, metabolism, and elimination. Different routes of administration (injection, oral) have different pharmacokinetics. The pharmacokinetics of compounded products have not been evaluated by the FDA and may differ from brand-name products.

Pharmacodynamics

The effects a medication produces in the body, including mechanism of action and side effects. The pharmacodynamics described in FDA labels come from studies of the FDA-approved products. Compounded products have not been studied the same way and may differ in formulation, purity, and potency.

Phentermine

A sympathomimetic medication (stimulant) approved by the FDA for short-term weight loss (typically 12 weeks or less). Different mechanism from GLP-1 medications. Phentermine is also part of an FDA-approved combination product with topiramate for longer-term use.

Potency

The concentration of active pharmaceutical ingredient in a medication. A compounded medication’s potency must match the prescribed dose. This is verified through testing (Certificate of Analysis). Potency is different from purity: a medication can be pure but have the wrong potency if improperly formulated.

Prior authorization

Insurance company requirement to approve a medication before covering it. A common barrier to branded GLP-1 access. Some telehealth programs help with prior authorization paperwork; others are cash-pay only and do not work with insurance.

Purity

The percentage of the product that is the desired active ingredient, free from contaminants, byproducts, or degradation. A Certificate of Analysis confirms purity. Purity is different from potency: a pure medication still needs to have the correct concentration (potency) of the active ingredient.

R

Receptor

A protein on the surface of or inside cells that binds to specific molecules (like hormones or medications) and triggers a cellular response. GLP-1 medications bind to GLP-1 receptors throughout the body, particularly in the gut, pancreas, and brain.

Receptor agonist

A medication that binds to a receptor and activates it. GLP-1 medications are agonists: they activate GLP-1 receptors. Different from antagonists, which block receptor activation.

S

Satiety

The feeling of fullness after eating. GLP-1 medications increase satiety by slowing gastric emptying and acting on appetite centers in the brain.

Semaglutide

The active ingredient in FDA-approved branded injectable and oral medications for type 2 diabetes and chronic weight management^[6]. Semaglutide is a GLP-1 receptor agonist. Some compounding pharmacies prepare compounded semaglutide, which is not FDA-approved and is not the same as the brand-name products.

Set point

The weight your body “prefers” to maintain. The set point is determined by genetics, past weight history, metabolic health, and lifestyle. The set point is regulated by hypothalamic appetite and satiety mechanisms. Some researchers think GLP-1 medications may lower the weight the body defends while you take them. Weight regain is common after stopping.

Sterile

Free from microorganisms (bacteria, fungi, viruses). Injectable medications must be sterile. Compounding pharmacies must follow strict sterile compounding protocols (USP 797 standards) to prepare injectable medications safely.

Subcutaneous injection

An injection administered into the fatty tissue just below the skin (between the dermis and muscle). The route of administration for injectable GLP-1 medications. Subcutaneous injections are less invasive than intravenous injections and allow for self-administration.

Surfactant

A substance that reduces surface tension, used in some medications to improve absorption or stability. Some compounding pharmacies use surfactants in their formulations; others do not. Ask your pharmacy about their formulation and whether surfactants are included.

T

Telogen effluvium

A temporary form of hair loss caused by physical stress, a significant change in weight over a short period, or nutritional deficiency. The most common type of hair shedding associated with GLP-1 medications. Typically resolves within 3-6 months. See detailed information at /patient-guide/body-changes/telogen-effluvium/.

Tirzepatide

The active ingredient in FDA-approved branded medications for type 2 diabetes and chronic weight management^[7]. A dual GLP-1/GIP receptor agonist, meaning it activates both GLP-1 and GIP receptors. Some compounding pharmacies prepare compounded tirzepatide, which is not FDA-approved and is not the same as the brand-name products.

Tolerance

See “Medication tolerance.”

Type 2 diabetes

A condition where cells become resistant to insulin and/or the pancreas cannot produce enough insulin to maintain normal blood sugar^[5]. The most common form of diabetes, accounting for about 90% of cases. GLP-1 medications improve blood sugar control and reduce A1c in type 2 diabetes.

Typical results

The average or expected outcome for a treatment in a population. When marketing medications or supplements, if testimonials or case studies show “atypical results” (better than average), Federal Trade Commission (FTC) guidance requires disclosure detailing what that actually means. Compounded medications have not undergone large clinical trials to establish the same results as branded versions.

U

USP 797 (United States Pharmacopeia Standard 797)

The standard for sterile compounding of injectable medications. Governs preparation of injectable compounded medications including GLP-1 vials. Covers requirements for facility design, staff training, air quality, sterilization, and testing. Licensed compounding pharmacies must comply with USP 797 to prepare injectable medications safely. Under USP 797, a multiple-dose vial is typically usable for up to 28 days after it is first punctured, or until the date on the label if that comes first^[10].

USP 800

The standard for handling hazardous drugs during compounding. While GLP-1 medications are not classified as hazardous, some compounding facilities use USP 800 guidelines for additional safety measures.

V

Visceral adiposity

Fat stored around internal organs in the abdominal cavity. More metabolically active and more strongly associated with cardiovascular and metabolic risk than subcutaneous fat (fat under the skin). Excess visceral adiposity is linked to insulin resistance, fatty liver disease, and inflammation. Trials that measured body composition have reported reductions in visceral fat with GLP-1 medications.

Visceral fat

See “Visceral adiposity.”

W

Waist circumference

The measurement around your waist at the level of the navel. Used as a marker of visceral adiposity. A waist circumference above 40 inches for men or 35 inches for women is associated with increased cardiovascular and metabolic risk.

Weight loss plateau

A period where weight loss slows or stops despite continuing medication and lifestyle changes. Plateaus are a normal part of weight loss. Some are temporary; on GLP-1 medications, weight loss also tends to level off after many months of treatment as the body adapts to a lower weight.

Citations

[1] National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). “Overweight and Obesity Definitions.” https://www.niddk.nih.gov/health-information/weight-management/overview

[2] Jastreboff AM, et al. “Tirzepatide Once Weekly for the Treatment of Obesity.” New England Journal of Medicine 2022;387:205-216. https://pubmed.ncbi.nlm.nih.gov/35658024/

[3] FDA. Wegovy (semaglutide) injection and tablets prescribing information (NDA 215256), boxed warning and section 5.1. Revised 06/2026. https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/215256s025lbl.pdf

[4] Eliasson B. “The Use of GLP-1 Receptor Agonists in Type 2 Diabetes.” Diabetes & Metabolism Journal 2011;35(2):99-111.

[5] American Diabetes Association. “Diagnosis and Classification of Diabetes Mellitus.” Diabetes Care 2013;36(Supplement 1):S67-S74.

[6] FDA. Wegovy (semaglutide) injection and tablets prescribing information (NDA 215256), revised 06/2026. https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/215256s025lbl.pdf

[7] FDA. Zepbound (tirzepatide) injection prescribing information (NDA 217806), revised 08/2026. https://www.accessdata.fda.gov/drugsatfda_docs/label/2026/217806Orig1s041correctedlbl.pdf

[8] Aronne LJ, et al. “Tirzepatide as Compared with Semaglutide for the Treatment of Obesity.” New England Journal of Medicine 2025;393(1):26-36. https://pubmed.ncbi.nlm.nih.gov/40353578/

[9] Lincoff AM, et al. “Semaglutide and Cardiovascular Outcomes in Obesity without Diabetes.” New England Journal of Medicine 2023;389(24):2221-2232. https://pubmed.ncbi.nlm.nih.gov/37952131/

[10] United States Pharmacopeia (USP). “General Chapter 797: Pharmaceutical Compounding - Sterile Preparations.” Revision published November 1, 2022; official November 1, 2023. https://www.usp.org/compounding/general-chapter-797

Wegovy is a trademark of Novo Nordisk A/S. Zepbound is a trademark of Eli Lilly and Company. Transformation Health is not affiliated with or endorsed by these companies.