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) is sourced from USP-grade suppliers and compounded by a licensed 503A or 503B pharmacy. 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. GLP-1 medications carry a black box warning for thyroid C-cell tumors (based on studies in animals)^[3]. This warning means there is a serious risk, but it does not mean the medication is prohibited – it means the risk must be discussed with your provider 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). Eligibility for GLP-1 weight management treatment requires BMI 30+, or 27+ with weight-related comorbidities^[1]. 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. Issued by an independent testing laboratory. A quality indicator for compounded medications. Reputable compounding pharmacies provide a COA with each compounded batch. See guidelines for choosing a safe compounding pharmacy at /cost/compounded-medications/choose-safe-compounding-pharmacy/.

Comorbidity

A health condition that exists alongside another. For GLP-1 eligibility, weight-related comorbidities include hypertension, type 2 diabetes, sleep apnea, cardiovascular disease, and others. If you have a comorbidity that is linked to your weight, you may qualify for GLP-1 treatment with a BMI as low as 27.

Compounded medication

A medication prepared by a licensed compounding pharmacy for an individual patient, using pharmaceutical-grade active ingredients. Compounded GLP-1 medications are not FDA-approved products. They are prepared under pharmacy compounding standards (503A or 503B) and are legal when a valid prescription is presented. 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. Absolute contraindications for GLP-1 medications include personal or family history of medullary thyroid carcinoma (MTC) or MEN 2. Relative contraindications (where the provider may still prescribe if benefits outweigh risks) include gastroparesis, pancreatitis history, and certain other conditions. 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). C-cell tumors are a type of thyroid cancer called medullary thyroid carcinoma (MTC). Animal studies showed that GLP-1 receptor agonists can cause C-cell tumors in rodents, which is why GLP-1 medications carry a black box warning for this risk in humans.

CGMP (Current Good Manufacturing Practice)

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

D

Dose escalation

The gradual increase of medication dose over time. GLP-1 medications are started at low doses (0.25 mg for semaglutide, 2.5 mg for tirzepatide) and increased at 4-week intervals (to 0.5 mg, 1 mg, 1.7 mg, or 2.4 mg for semaglutide; to 5 mg, 7.5 mg, 10 mg, or 15 mg for tirzepatide) to reduce GI side effects and allow the body to adjust. 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. There is a rare risk of DKA with GLP-1 use, particularly in people with type 1 diabetes. This is why type 1 diabetes is generally considered a contraindication to GLP-1 medications.

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). The FDA approves medications based on efficacy demonstrated in clinical trials. Compounded medications have not undergone the same clinical trial testing as FDA-approved branded versions.

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. Your independent provider may consult with endocrinologists on complex cases.

F

Food noise

Informal term for constant, intrusive thoughts about food, cravings, and eating. Often experienced by people with insulin resistance or certain appetite regulation differences. GLP-1 medications reduce food noise by acting on appetite centers in the brain.

FSA (Flexible Spending Account)

A pre-tax employer benefit account that can be used for eligible healthcare expenses, including prescription medications. GLP-1 medications are FSA-eligible, which means you can pay for them with pre-tax dollars, reducing your taxable income.

Fungal contamination

Contamination of a compounded medication with fungal organisms during preparation. A serious quality failure that makes a compounded medication unsafe. Reputable compounding pharmacies follow strict sterility protocols and test batches to prevent contamination. Check that your pharmacy provides a Certificate of Analysis with each batch.

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. Gastroparesis (excessively slow gastric emptying) is a contraindication to GLP-1 use because GLP-1 medications further slow emptying.

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. Dual agonists may produce greater weight loss and metabolic improvement than GLP-1-only medications in some patients.

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. A contraindication to GLP-1 medications, which further slow gastric emptying. 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. In the Transformation Health program, an independent, US-licensed provider evaluates your health history and determines 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.

HSA (Health Savings Account)

A pre-tax savings account paired with a high-deductible health plan, used for eligible medical expenses including prescription medications. HSAs have higher contribution limits than FSAs and unused funds can roll over year to year, making them advantageous for ongoing medication costs.

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 third-party accreditation and compliance verification service for online pharmacies and telehealth providers. LegitScript certification provides assurance that a telehealth company or pharmacy follows state and federal regulations and legitimate healthcare practices.

Lipolysis

The breakdown of fat stored in adipose tissue into free fatty acids that can be used for energy. GLP-1 medications increase lipolysis partly through weight loss and partly through direct metabolic effects.

M

MACE (Major Adverse Cardiovascular Events)

A composite cardiovascular outcome measure used in clinical trials, typically including heart attack, stroke, and cardiovascular death. GLP-1 medications have demonstrated cardiovascular benefits in large clinical trials, reducing MACE in patients with existing cardiovascular disease or diabetes.

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, your provider will discuss screening before considering 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. GLP-1 medications are prescribed to treat obesity when BMI 30+ or BMI 27+ with weight-related comorbidities.

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 branded medication is approved for the active ingredient but not for that specific indication.

P

PCAB (Pharmacy Compounding Accreditation Board)

A voluntary accreditation body for compounding pharmacies, confirming standards above the regulatory minimum. PCAB-accredited pharmacies have demonstrated expertise in quality assurance, sterile compounding, and staff training. Choosing a PCAB-accredited pharmacy is a good sign of quality. See guidance on selecting a safe pharmacy at /cost/compounded-medications/choose-safe-compounding-pharmacy/.

Pancreatitis

Inflammation of the pancreas. There is a rare risk of pancreatitis with GLP-1 use. If you have a history of pancreatitis, your provider will discuss whether GLP-1 is appropriate for you.

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. Compounded medications should have similar pharmacokinetics to branded versions with the same active ingredient, but this has not been independently verified by the FDA.

Pharmacodynamics

The effects a medication produces in the body, including mechanism of action and side effects. GLP-1 pharmacodynamics are the same whether the medication is branded or compounded (as long as the active ingredient is the same).

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. Often combined with other medications for longer-term use, though regulatory approval is limited.

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. Transformation Health’s independent providers work directly with your insurance to secure prior authorization if your plan requires it.

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]. Also available as a compounded formulation through licensed US-based pharmacies. Semaglutide is a GLP-1 receptor agonist.

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. GLP-1 medications work partly by resetting your metabolic set point, allowing sustainable weight loss at a lower body weight.

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. Also available as a compounded formulation through licensed US-based pharmacies.

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 of what “typical results” are. Compounded medications have not undergone the large clinical trials that establish typical results for 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.

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. GLP-1 medications preferentially reduce visceral adiposity.

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 and typically last 2-4 weeks before weight loss resumes. Plateaus occur because your body is defending your current weight as a new set point.

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. “Prescribing Information for semaglutide for type 2 diabetes and chronic weight management: Black Box Warning.” Official prescribing information. 2021-2022. https://www.accessdata.fda.gov/drugsatfda_docs/label/2021/215256s000lbl.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. “Prescribing Information for semaglutide for chronic weight management.” Official prescribing information. 2021. https://www.accessdata.fda.gov/drugsatfda_docs/label/2021/215256s000lbl.pdf

[7] FDA. “Prescribing Information for tirzepatide for chronic weight management.” Official prescribing information. 2023. https://www.accessdata.fda.gov/drugsatfda_docs/label/2023/217806s000lbl.pdf