Mechanism of Action
Semaglutide is a selective GLP-1 receptor agonist that mimics the incretin hormone GLP-1. When it binds to GLP-1 receptors, it stimulates glucose-dependent insulin secretion, suppresses glucagon release, slows gastric emptying, and acts on hypothalamic appetite centers to reduce hunger and caloric intake.¹ It has a half-life of approximately 7 days, allowing once-weekly dosing.
Tirzepatide is a dual-acting agonist that activates both GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) receptors. The addition of GIP signaling is thought to enhance insulin sensitivity beyond GLP-1 alone, improve fat metabolism, and potentially provide additional appetite suppression.² This dual mechanism is the primary explanation for tirzepatide's superior weight loss results in clinical trials.
Weight Loss Efficacy
In the STEP trials, semaglutide 2.4mg produced approximately 14.9% total body weight loss over 68 weeks, with some participants achieving over 20% loss.¹ In the SURMOUNT-1 trial, tirzepatide at the highest dose (15mg) produced 22.5% weight loss over 72 weeks — roughly 50% more weight loss than semaglutide at its maximum approved weight management dose.²
The SURPASS trials (which compared tirzepatide against semaglutide 1mg for type 2 diabetes) showed tirzepatide was superior in both A1C reduction and weight loss at all dose levels tested.³ These results make tirzepatide the most effective anti-obesity medication ever evaluated in Phase III clinical trials to date.
Side Effect Profiles
Both medications share a similar side effect profile characteristic of GLP-1 agonists. Gastrointestinal effects — primarily nausea, vomiting, diarrhea, and constipation — are the most common adverse events. These are typically most pronounced during the dose-titration phase and generally diminish over time.¹ ²
In clinical trials, tirzepatide showed comparable or slightly lower rates of nausea compared to semaglutide at equivalent efficacy doses, which some researchers attribute to the GIP receptor co-agonism potentially buffering GI side effects.³ Both medications carry warnings regarding pancreatitis, thyroid C-cell tumors (based on rodent studies), and gallbladder-related events.
Cost and Accessibility
Both medications are premium-priced. Wegovy (semaglutide for weight loss) has a US list price of approximately $1,350/month, while Zepbound (tirzepatide for weight loss) lists at approximately $1,060/month.⁴ However, actual out-of-pocket costs depend heavily on insurance coverage, which varies by plan and indication. Generic versions are not yet available for either compound.
Semaglutide has a broader range of available formulations, including injectable (Ozempic, Wegovy) and oral (Rybelsus, and the recently approved oral Wegovy pill).⁵ Tirzepatide is currently available only as a weekly injection, though oral formulations are in clinical development.