GLP-1 vs Bariatric Surgery: Semaglutide, Tirzepatide, and When Surgery Still Wins

How much weight do semaglutide and tirzepatide produce?

In STEP 1, 1,961 adults with obesity or overweight and no diabetes lost a mean 14.9% of body weight on semaglutide 2.4 mg weekly over 68 weeks, versus 2.4% on placebo [3]. In SURMOUNT-1, 2,539 adults with the same profile lost 15.0%, 19.5% and 20.9% on tirzepatide 5, 10 and 15 mg over 72 weeks, versus 3.1% on placebo [4]. STEP 2, which enrolled 1,210 adults with type 2 diabetes, reported a mean 9.6% weight loss on semaglutide 2.4 mg versus 3.4% on placebo at 68 weeks, against 14.9% in the separate STEP 1 trial that enrolled adults without diabetes; the two trials enrolled different populations [7][3].

The two drugs have been compared directly. SURMOUNT-5 randomized 751 adults with obesity and no diabetes to the maximum tolerated dose of tirzepatide (10 or 15 mg) or semaglutide (1.7 or 2.4 mg) for 72 weeks: weight fell 20.2% versus 13.7%, and waist circumference 18.4 cm versus 13.0 cm [1]. Tirzepatide activates both the GIP and GLP-1 receptors while semaglutide activates GLP-1 alone, and whether the added GIP activity explains the gap is not established by these trials [8][1].

The doses used in these trials were once-weekly injections. Wegovy also comes as a once-daily tablet [9]. The Wegovy injection label starts patients at 0.25 mg weekly for four weeks, then titrates every four weeks to a usual maintenance dose of 2.4 mg [9]. Wegovy injection is indicated for weight reduction in adults and in children aged 12 and older with obesity [9].

How much weight does bariatric surgery produce?

The randomized surgical comparison here is the five-year follow-up of 150 adults with type 2 diabetes and a BMI of 27 to 43, assigned to intensive medical therapy alone or to intensive medical therapy plus Roux-en-Y gastric bypass or sleeve gastrectomy. At five years, body weight had fallen 23% after gastric bypass and 19% after sleeve gastrectomy, against 5% with medical therapy alone [2].

Those are five-year weights from a trial of people with type 2 diabetes; the semaglutide and tirzepatide figures above come from 68- to 72-week trials in other populations [1][3][4][2].

Diabetes control: what the five-year surgery trial showed

The surgical trial's primary endpoint was an HbA1c of 6.0% or less at five years, with or without diabetes medication. Of the 134 patients who completed five years of follow-up, it was met by 2 of 38 (5%) on intensive medical therapy, 14 of 49 (29%) after gastric bypass and 11 of 47 (23%) after sleeve gastrectomy; the unadjusted P values were 0.01 and 0.03, and in the intention-to-treat analysis the differences did not reach significance (P=0.08 for gastric bypass, P=0.17 for sleeve gastrectomy) [2]. Mean HbA1c fell 2.1 points with surgery versus 0.3 with medical therapy (P=0.003). Triglycerides fell 40%, 29% and 8%, and insulin use fell 35%, 34% and 13%, across the gastric bypass, sleeve and medical groups [2].

The drug trials measure a different thing. In SURPASS-2, 1,879 adults with type 2 diabetes took tirzepatide 5, 10 or 15 mg or semaglutide 1 mg for 40 weeks; HbA1c fell 2.01, 2.24 and 2.30 points on tirzepatide and 1.86 points on semaglutide [10]. Those are average reductions during continued treatment, while the surgical trial's headline result counts people who reached a target [10][2].

What the 2019 guideline says about surgery

The 2019 clinical practice guideline from AACE, The Obesity Society, ASMBS, the Obesity Medicine Association and the American Society of Anesthesiologists concludes that bariatric procedures remain a safe and effective intervention for higher-risk patients with obesity, and that clinical decision-making should be evidence-based within the context of a chronic disease [11].

Where a GLP-1 drug has its own evidence

For adults aged 45 or older with pre-existing cardiovascular disease and a BMI of 27 or more but no diabetes, SELECT found fewer cardiovascular deaths, heart attacks or strokes with semaglutide 2.4 mg than with placebo: 6.5% versus 8.0% (HR 0.80) [5]. Participants were followed for a mean of 39.8 months, and mean exposure to semaglutide or placebo was 34.2 months [5]. SELECT did not compare semaglutide with surgery [5].

Stopping a GLP-1 drug

In STEP 4, 803 adults who had reached semaglutide 2.4 mg were randomized to continue it or switch to placebo for 48 weeks; weight fell another 7.9% on semaglutide and rose 6.9% on placebo [6]. STEP 4 tested semaglutide only.

What the labels list as risks

Both Wegovy and Zepbound carry a boxed warning for thyroid C-cell tumors in rodents, with human relevance unknown, and are contraindicated in people with a personal or family history of medullary thyroid carcinoma or MEN 2 [9][8]. The Wegovy label warns about acute pancreatitis and says treatment should be discontinued if pancreatitis is suspected; it also lists acute gallbladder disease, severe gastrointestinal reactions with a statement that the drug is not recommended in patients with severe gastroparesis, acute kidney injury from volume depletion, hypoglycemia when combined with insulin or a sulfonylurea, and a mean resting heart rate rise of 1 to 4 bpm [9].

In the Wegovy weight-management trials, nausea was reported by 44% versus 16% on placebo, diarrhea by 30% versus 16%, vomiting by 24% versus 6% and constipation by 24% versus 11%; 6.8% of semaglutide patients and 3.2% of placebo patients stopped treatment because of adverse reactions [9]. For tirzepatide 15 mg the pooled rates were nausea 28% versus 8%, vomiting 13% versus 2%, diarrhea 23% versus 8% and constipation 11% versus 5%, with 6.7% versus 3.4% stopping [8]. Rates from different trials are not directly comparable [9].

The Wegovy label also warns that pulmonary aspiration during general anesthesia or deep sedation has been reported in patients receiving GLP-1 receptor agonists, and says patients should report any planned surgery or procedure [9].

Can these drugs be used after bariatric surgery?

The Wegovy indication text covers weight reduction in adults with obesity, or with overweight plus at least one weight-related condition, alongside a reduced-calorie diet and increased physical activity, and does not mention prior bariatric surgery [9]. The label's warnings on aspiration under anesthesia, pancreatitis and severe gastroparesis apply to patients taking the drug [9].