STEP 5 Trial: Two-Year Semaglutide Weight Loss Results Explained

What Is the STEP 5 Trial and Why Does It Matter?

STEP 5 is the longest dedicated randomized controlled trial of semaglutide 2.4 mg for chronic weight management, running to 104 weeks (two years) in adults without type 2 diabetes. Where the earlier trials capped out at 68 weeks, STEP 5 answered the question that every clinician and patient actually wants answered: does the weight loss hold? The answer is yes, with 15.2% mean body-weight reduction in the semaglutide group versus 2.6% in the placebo group at week 104 [1].

The trial enrolled 304 participants across multiple sites, all with a BMI ≥30 or a BMI ≥27 plus at least one weight-related comorbidity such as hypertension or dyslipidemia [1]. Participants had no diagnosis of type 2 diabetes, distinguishing this population from STEP 2. Both arms received counseling on a reduced-calorie diet and increased physical activity throughout, so the drug effect sits on top of structured lifestyle support, not as a replacement for it [1].

Cardiometabolic markers also improved over two years. Waist circumference fell by a mean of 13.8 cm in the semaglutide arm versus 3.4 cm in the placebo arm [1]. Systolic blood pressure, fasting plasma glucose, and HbA1c all trended favorably [1]. These secondary endpoints matter because the FDA's 2024 Wegovy label now carries a cardiovascular outcomes indication based on the broader semaglutide evidence base [2].

The most common adverse events were gastrointestinal: nausea (44.2% semaglutide vs 16.0% placebo), diarrhea (32.0% vs 14.9%), and vomiting (24.5% vs 6.8%) [1]. Serious adverse events were reported in 9.8% of the semaglutide arm and 6.4% of the placebo arm over two years [1]. Discontinuation due to adverse events occurred in 7.7% of semaglutide-treated participants [1].

STEP 1 Trial: The 68-Week Foundation

STEP 1 established the benchmark dose-and-duration profile that STEP 5 then extended. Published in the New England Journal of Medicine in March 2021, STEP 1 randomized 1,961 adults with obesity (no T2D) to semaglutide 2.4 mg or placebo for 68 weeks [3].

Mean weight loss was 14.9% with semaglutide versus 2.4% with placebo [3]. That is a difference of 12.4 percentage points, achieved with the same once-weekly subcutaneous injection protocol later used in STEP 5. The proportion of participants achieving ≥5% weight loss was 86.4% on semaglutide versus 31.5% on placebo [3]. More than half the semaglutide group (50.5%) lost at least 15% of body weight [3].

The NEJM authors noted: "Participants who received semaglutide had a mean change in body weight of -14.9% as compared with -2.4% for those who received placebo" [3]. Waist circumference, blood pressure, HbA1c, lipids, and C-reactive protein all improved relative to placebo [3]. The STEP 1 safety profile matched what would later appear in STEP 5: predominantly gastrointestinal events, mostly mild to moderate in severity, concentrated during the 16-week dose-escalation phase [3].

STEP 1 is the primary evidence base cited in the FDA's Wegovy approval package, and the 2024 revised label references both the 68-week efficacy and the two-year maintenance data from STEP 5 [2]. For prescribers deciding between semaglutide and tirzepatide, SURMOUNT-1 (N=2,539) provides the parallel benchmark for tirzepatide 15 mg, which produced 20.9% mean weight loss at 72 weeks in adults without T2D [4]. Comparing STEP 1 to SURMOUNT-1 is an indirect comparison only; no head-to-head non-inferiority trial between semaglutide 2.4 mg and tirzepatide 15 mg has been published as of this writing.

STEP 2 Trial: Semaglutide in Type 2 Diabetes

Adults with type 2 diabetes lose less weight on semaglutide than those without the condition. STEP 2 (N=1,210, published in The Lancet, March 2021) randomized participants with T2D and a BMI ≥27 to semaglutide 2.4 mg, semaglutide 1.0 mg, or placebo for 68 weeks [5].

Mean weight loss with semaglutide 2.4 mg was 9.6% versus 3.4% for placebo [5]. The 1.0 mg dose produced 7.0% mean weight loss [5]. HbA1c fell by 1.6 percentage points in the 2.4 mg group versus 0.4 percentage points in the placebo group [5]. The gap between the T2D population in STEP 2 and the non-T2D population in STEP 1 (9.6% vs 14.9%) is consistent with what clinicians see across the GLP-1 class: insulin resistance and beta-cell dysfunction blunt the anorectic response [5].

SURMOUNT-2 (N=938, The Lancet, June 2023) provides the tirzepatide parallel in T2D: mean weight loss of 15.7% with tirzepatide 15 mg versus 3.3% placebo at 72 weeks [6]. That figure substantially exceeds STEP 2's semaglutide 2.4 mg result, though again the trials differ in design, baseline BMI, and background therapy [6].

The AACE/ACE obesity clinical practice guidelines specify that pharmacotherapy is appropriate for patients with a BMI ≥30 or ≥27 with at least one weight-related comorbidity, and they explicitly list GLP-1 receptor agonists as a first-line pharmacological option [7]. STEP 2's HbA1c and weight data together support using semaglutide 2.4 mg in T2D patients who need both glycemic and weight management, provided renal function and gastrointestinal tolerability permit [5][7].

STEP 3 Trial: Semaglutide Plus Intensive Behavioral Therapy

Adding intensive behavioral therapy (IBT) to semaglutide 2.4 mg does not dramatically amplify weight loss beyond semaglutide alone. STEP 3 (N=611, published in JAMA, April 2021) tested semaglutide 2.4 mg plus IBT (30 counseling visits, meal replacements for the first 8 weeks) against placebo plus IBT for 68 weeks in adults without T2D [8].

Mean weight loss was 16.0% with semaglutide plus IBT versus 5.7% with placebo plus IBT [8]. Comparing STEP 3's 16.0% to STEP 1's 14.9% shows that the behavioral augmentation added roughly 1.1 percentage points of additional weight loss when both arms received the same intensive support [8][3]. That is a clinically modest increment. The proportion achieving ≥5% weight loss was 86.6% (semaglutide plus IBT) versus 47.6% (placebo plus IBT) [8].

The JAMA authors concluded: "Among adults with overweight or obesity, 68-week treatment with once-weekly subcutaneous semaglutide 2.4 mg plus intensive behavioral therapy resulted in significantly greater weight loss compared with placebo plus IBT" [8]. The safety profile was consistent with STEP 1: nausea and vomiting were the dominant adverse events, concentrated in the dose-escalation phase [8].

For patients with access to structured behavioral support programs, STEP 3's data suggest IBT remains worth offering. The absolute benefit of adding IBT to semaglutide is smaller than adding semaglutide to IBT. The placebo plus IBT arm lost 5.7% versus STEP 1's placebo arm losing only 2.4%, confirming IBT adds roughly 3 percentage points of weight loss on its own [8][3].

STEP 8 Trial: Semaglutide vs Liraglutide Head-to-Head

Semaglutide 2.4 mg produces roughly twice the weight loss of liraglutide 3.0 mg over 68 weeks. STEP 8 (N=338, published in JAMA, January 2022) is the only head-to-head randomized trial directly comparing the two subcutaneous GLP-1 agonists approved for chronic weight management [9].

Mean weight loss at 68 weeks was 15.8% with semaglutide 2.4 mg versus 6.4% with liraglutide 3.0 mg [9]. The between-group difference of 9.4 percentage points was statistically significant (P<0.001) [9]. More semaglutide-treated participants achieved ≥10% weight loss (70.9% vs 25.6%) and ≥20% weight loss (38.5% vs 6.0%) [9].

Discontinuation due to adverse events was higher with liraglutide (13.5%) than with semaglutide (3.1%) in this trial, despite liraglutide's being injected daily versus semaglutide's once-weekly regimen [9]. Gastrointestinal events were numerically more frequent with semaglutide but did not translate into higher dropout [9]. These findings have clinical formulary implications: where both agents are available, the STEP 8 data give a strong quantitative rationale for choosing semaglutide 2.4 mg over liraglutide 3.0 mg [9].

Liraglutide 3.0 mg (Saxenda) still holds its FDA approval for chronic weight management, and some patients tolerate daily liraglutide better than weekly semaglutide injections. The 2024 Wegovy label does not reference STEP 8 directly, but the prescribing information for both agents is available on the FDA's accessdata portal [2].

Two-Year Durability: What STEP 5 Adds That Shorter Trials Cannot

Sixty-eight weeks is long enough to demonstrate efficacy. Two years is long enough to begin characterizing durability. That distinction matters because obesity is a chronic condition requiring long-term pharmacotherapy.

STEP 5's 104-week data show that the rate of weight loss plateaued around week 60 and then stabilized [1]. The semaglutide group did not regain weight during weeks 68 to 104; mean body weight stayed within 1 percentage point of the week-60 nadir [1]. The placebo group also remained relatively stable across that window, meaning both groups reached a new equilibrium and held it [1].

The SURMOUNT-4 trial (N=783, JAMA, October 2024) provides a useful contrast for tirzepatide: participants who completed 36 weeks of tirzepatide open-label lead-in and then were randomized to continue tirzepatide maintained their weight loss (additional 5.5% reduction) while those switched to placebo regained 14.8% [10]. The STEP 5 design did not include a withdrawal arm, so direct comparison is not possible, but SURMOUNT-4 confirms the general principle that GLP-1 class drugs require continued dosing for sustained effect [10].

For the treating clinician, two-year data from STEP 5 support writing prescriptions with an explicit long-term plan rather than a defined "course" of therapy. Weight regain after stopping semaglutide has been documented in the STEP 1 extension study, where participants regained approximately two-thirds of lost weight within one year of discontinuation [3].

The following decision framework integrates the four STEP trials plus STEP 8 into a prescribing logic that the HealthRX medical team uses internally for patient triage. Patients without T2D and BMI ≥30 are offered semaglutide 2.4 mg as a first-line agent with STEP 1 and STEP 5 as the primary efficacy references. Patients with T2D have STEP 2 cited explicitly at consent. Patients with access to structured behavioral therapy programs are counseled using STEP 3's marginal-benefit data, so expectations are calibrated. Patients switching from liraglutide 3.0 mg are counseled with STEP 8's between-group delta (9.4 percentage points additional weight loss) as a concrete expected benefit of the switch.

Cardiovascular Outcomes: The SELECT Trial in Context

STEP 5 was not powered for cardiovascular endpoints. The SELECT trial (N=17,604, NEJM, November 2023) fills that gap [11].

SELECT enrolled adults with established cardiovascular disease, a BMI ≥27, and no diabetes. Semaglutide 2.4 mg reduced the rate of major adverse cardiovascular events (MACE: cardiovascular death, non-fatal MI, non-fatal stroke) by 20% relative to placebo over a mean follow-up of 39.8 months (HR 0.80 to 95% CI 0.72 to 0.90, P<0.001) [11]. That is a landmark finding for a non-diabetes population and directly influenced the 2024 Wegovy label update, which now includes reduction of cardiovascular events as an approved indication [2][11].

The SELECT population's mean weight loss was 9.4% at 104 weeks with semaglutide versus 0.9% with placebo [11]. The MACE benefit appeared to precede the full weight-loss effect, with event-curve separation visible by week 12, suggesting mechanisms beyond weight reduction alone, including anti-inflammatory effects and direct myocardial action [11]. The NEJM editors noted in their summary: "The treatment effect was consistent across subgroups defined according to body-mass index, age, sex, race or ethnic group, and geographic region" [11].

Clinicians prescribing semaglutide 2.4 mg to patients with prior MI, stroke, or established coronary artery disease can now cite SELECT alongside STEP 5 to justify two years or more of continued therapy under both weight-management and cardiovascular-risk-reduction indications [2][11].

Tirzepatide vs Semaglutide: Where the SURMOUNT Trials Fit

Tirzepatide (Zepbound, Mounjaro) activates both GIP and GLP-1 receptors. The SURMOUNT program is the tirzepatide analog of the STEP program.

SURMOUNT-1 (N=2,539, NEJM, July 2022) found that tirzepatide 15 mg produced 20.9% mean weight loss at 72 weeks in adults without T2D, versus 3.1% placebo [4]. SURMOUNT-3 (N=579, Nature Medicine, November 2023) tested tirzepatide after a 12-week intensive lifestyle run-in and found 18.4% additional weight loss with tirzepatide versus 2.5% with placebo [12]. SURMOUNT-2 (N=938, The Lancet, June 2023) showed 15.7% weight loss with tirzepatide 15 mg in T2D versus 3.3% placebo [6].

The Zepbound FDA label (2024 revision) includes weight-management and, separately, obstructive sleep apnea indications [13]. The Wegovy label (2024 revision) includes weight management and cardiovascular event reduction [2].

No head-to-head randomized trial of semaglutide 2.4 mg versus tirzepatide 15 mg has been published. The SURMOUNT program's larger weight-loss numbers are consistent across multiple trials, but cross-trial comparison is limited by different populations, titration schedules, and trial durations. A prescriber choosing between agents should weigh efficacy signals, patient tolerability, formulary access, and cardiovascular history [2][13][7].

Dose Escalation, Tolerability, and Long-Term Prescribing Logistics

Semaglutide 2.4 mg is reached via a four-step escalation: 0.25 mg weekly for weeks 1 to 4, then 0.5 mg for weeks 5 to 8, then 1.0 mg for weeks 9 to 12, then 1.7 mg for weeks 13 to 16, then 2.4 mg from week 17 onward [2]. Patients who do not tolerate a dose escalation may remain at the prior dose for an additional four weeks before attempting the step up again [2].

Across STEP 1, 3, 5, and 8, nausea was the most frequently reported adverse event, appearing in 40% to 50% of semaglutide-treated participants [3][8][1][9]. The majority of nausea was mild to moderate and resolved within the first eight to twelve weeks [1]. Serious adverse events of special interest include acute pancreatitis (reported in <1% across STEP trials), gallbladder disease (cholelithiasis: approximately 2.2% semaglutide vs 0.7% placebo in STEP 1), and heart rate increase (mean +1 to +3 bpm) [3][2].

The 2024 Wegovy label carries a boxed warning for thyroid C-cell tumors based on rodent data; semaglutide is contraindicated in patients with personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2 [2]. Renal and hepatic dose adjustment is not required per the current label, but caution is advised in severe impairment [2].

For long-term logistics, the STEP 5 two-year completion rate was 75.3% in the semaglutide arm and 71.7% in the placebo arm [1], which is high for a two-year obesity pharmacotherapy trial and reflects the tolerability and patient acceptance of the once-weekly injection regimen [1].