Comprehensive answers to the most common questions about semaglutide, tirzepatide, and retatrutide — covering mechanisms, weight loss data, side effects, safety, and sourcing.
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GLP-1 peptides are glucagon-like peptide-1 receptor agonists — compounds that mimic the GLP-1 hormone naturally produced in the gut after eating. They suppress appetite, slow gastric emptying, and regulate blood sugar. The three main research-grade GLP-1 compounds are semaglutide (GLP-1 only), tirzepatide (GLP-1 + GIP), and retatrutide (GLP-1 + GIP + glucagon).
GLP-1 peptides reduce appetite by activating GLP-1 receptors in the hypothalamus, signaling fullness. They also slow gastric emptying, prolonging satiety after meals. Higher-generation compounds (tirzepatide, retatrutide) add GIP and glucagon receptor agonism, which increases thermogenesis and energy expenditure beyond appetite suppression alone.
GLP-1 (glucagon-like peptide-1) regulates appetite, insulin secretion, and gastric emptying. GLP-2 (glucagon-like peptide-2) primarily regulates intestinal growth and nutrient absorption. They are different hormones with different receptors and functions. GLP-1 agonists are used for metabolic research; GLP-2 agonists are used for intestinal research.
No. GLP-1 peptides are not insulin. They stimulate insulin secretion from the pancreas in a glucose-dependent manner (only when blood sugar is elevated), but they are a different class of compound. GLP-1 agonists also suppress glucagon, slow gastric emptying, and reduce appetite — effects insulin does not have.
Yes. Ozempic is a brand name for semaglutide manufactured by Novo Nordisk. Wegovy is also semaglutide, approved at a higher dose (2.4 mg/week) specifically for weight management. Research-grade semaglutide is the same active molecule available for research purposes.
STEP 1 reported a mean loss of approximately 15% of body weight at 68 weeks at the 2.4 mg/week dose in trial participants, with wide individual variance (roughly 5% to over 20%) and larger losses at higher starting BMI. These are published trial figures, not expectations for any research material.
STEP 4 extension data shows subjects regain approximately 2/3 of lost weight within 1 year of stopping semaglutide. The underlying metabolic drivers of obesity are not permanently resolved — GLP-1 receptor activity returns to baseline. Maintenance dosing or transition to a higher-generation compound is typically needed.
Long-term safety data for semaglutide extends to 2+ years in clinical trials with no new safety signals. The FLOW trial showed cardiovascular benefit in high-risk populations with long-term use. The practical limitation is cost and access, not safety. Maintenance dosing appears to be required to sustain weight loss.
Clinical data shows tirzepatide produces greater weight loss than semaglutide — approximately 22% vs 15% of body weight. The SURPASS-CVOT head-to-head trial confirmed tirzepatide's superiority on weight loss endpoints. However, semaglutide has more published cardiovascular outcome data. 'Better' depends on the research goal.
Mounjaro (tirzepatide) and Ozempic (semaglutide) are different molecules. Ozempic targets only the GLP-1 receptor. Mounjaro targets both GLP-1 and GIP receptors — the dual mechanism produces greater weight loss (~22% vs ~15%). Mounjaro is manufactured by Eli Lilly; Ozempic by Novo Nordisk.
Appetite suppression begins within the first week. Measurable weight loss (1.5–3 kg) typically occurs within 4 weeks. The most rapid weight loss phase is between weeks 4–36. Maximum weight loss (~22% of body weight) is achieved at approximately 72 weeks in SURMOUNT-1.
Hair loss (telogen effluvium) is reported by some subjects on tirzepatide and semaglutide. It is not a direct drug effect — it is caused by rapid weight loss and caloric restriction, which triggers the hair growth cycle to pause. It is typically temporary and resolves within 3–6 months as weight loss stabilizes.
Retatrutide is a triple GLP-1, GIP, and glucagon receptor agonist developed by Eli Lilly. It is the most potent GLP-class compound studied to date, producing approximately 24% weight loss at 48 weeks in Phase 2 trials. It is currently in Phase 3 trials and is not FDA approved. Research-grade retatrutide is available from suppliers like Purgo Labs.
No. Retatrutide completed Phase 2 trials with strong results and is currently in Phase 3 trials. It is not FDA approved for any indication. Semaglutide and tirzepatide are the only GLP-class compounds with FDA approval.
Retatrutide adds glucagon receptor agonism on top of tirzepatide's GLP-1 + GIP mechanism. This adds thermogenesis and increased energy expenditure. Phase 2 data shows retatrutide produced ~24% weight loss at 48 weeks vs tirzepatide's ~22% at 72 weeks. Retatrutide has higher GI side effect rates and less published safety data.
The most common side effects are gastrointestinal: nausea (47–58% at therapeutic doses), vomiting (20–35%), diarrhea (15–25%), and constipation (10–15%). These are higher than semaglutide or tirzepatide due to the glucagon receptor component. Slow titration significantly reduces side effects.
The most common GLP-1 side effects are gastrointestinal: nausea (10–58% depending on compound and dose), vomiting (8–35%), diarrhea (12–30%), and constipation (10–24%). These are most pronounced during dose escalation and typically resolve within 8–12 weeks.
Pancreatitis is a rare but serious class-level risk for GLP-1 agonists. The absolute risk is very low (<0.1% in clinical trials). Subjects with a history of pancreatitis or gallbladder disease should not use GLP-1 compounds. Persistent severe abdominal pain requires immediate discontinuation.
GLP-1 peptides cause weight loss from both fat and lean mass. STEP trials showed approximately 40% of weight lost on semaglutide was lean mass — similar to other caloric restriction methods. Adequate protein intake (1.2–1.6g/kg/day) and resistance training significantly reduce lean mass loss during GLP-1 research.
GLP-1 agonists carry a class warning about thyroid C-cell tumors based on rodent studies. This effect has not been observed in human clinical trials. However, GLP-1 compounds are contraindicated in subjects with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2).
Purgo Labs carries research-grade semaglutide, tirzepatide, and retatrutide with third-party COAs confirming purity. All compounds are sold for research purposes only. Code HEALTH is 20% off every order at Purgo Labs (verified recurring).
Key quality indicators: third-party Certificate of Analysis (COA) from an independent lab, HPLC purity testing (≥98%), accurate vial weights, proper cold-chain shipping, and transparent sourcing. Avoid suppliers who don't publish COAs or who make human use claims.
GLP-1 peptides are typically supplied as lyophilized (freeze-dried) powder. Reconstitution involves adding bacteriostatic water (BAC water) to the vial using a sterile syringe. The standard reconstitution ratio depends on the vial size and desired concentration. Reconstituted peptides should be stored at 2–8°C and used within 28–30 days.
Research-grade GLP-1 peptides are legal to purchase for research purposes in the United States, Canada, Australia, and most of Europe. They are not approved for human use and cannot be sold as dietary supplements or medications. Legal status varies by jurisdiction — see our compound-specific legal status pages for details.
Purgo Labs carries research-grade semaglutide, tirzepatide, and retatrutide with third-party COAs. Use code HEALTH for 20% off.
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