STEP-1 Results in Detail: Numbers, Subgroups, and Time Course

At a glance

Parameter	Detail
Trial name	STEP-1 (Semaglutide Treatment Effect in People with Obesity)
N	1,961 (semaglutide 1,306; placebo 655)
Population	Adults with BMI ≥30 kg/m² (or ≥27 with ≥1 weight-related comorbidity), without diabetes
Intervention	Subcutaneous semaglutide 2.4 mg once weekly, dose-escalated over 16 weeks
Comparator	Matched placebo injection, same escalation schedule
Background	Both groups received lifestyle intervention (500 kcal/day deficit counseling, ≥150 min/week physical activity)
Duration	68-week treatment period
Primary endpoint	Percent change in body weight from baseline at week 68
Co-primary endpoint	Proportion achieving ≥5% body weight loss at week 68
Key result	−14.9% vs −2.4% body weight change (P <0.001)

The Primary Endpoint: More Than Just a Mean

The headline number from STEP-1 is familiar to most clinicians by now: −14.9% mean body weight change with semaglutide versus −2.4% with placebo. But the statistical framing matters. The estimated treatment difference was −12.4 percentage points (95% CI, −13.4 to −11.5; P <0.001), analyzed using a mixed model for repeated measures (MMRM) under the "treatment policy" estimand, which included all randomized participants regardless of whether they stayed on drug.

A second "trial product" estimand, which modeled the effect assuming full adherence, yielded a larger treatment difference: −15.3% versus −2.6%. The gap between these two numbers (roughly one percentage point) tells you something about the real-world penalty of discontinuation. About 11% of participants in the semaglutide group stopped treatment early, mostly for gastrointestinal side effects. Even after folding those dropouts into the analysis, the primary result held with overwhelming statistical confidence.

The co-primary endpoint was categorical: 86.4% of semaglutide-treated participants lost ≥5% body weight versus 31.5% on placebo (odds ratio, 11.22; 95% CI, 8.88 to 14.19). The 5% threshold is the FDA's traditional bar for approval of anti-obesity medications, and semaglutide cleared it in the vast majority of participants receiving active drug.

Secondary Endpoints and the Responder Cascade

What made STEP-1 stand out from earlier anti-obesity medication trials was the distribution of high responders. The trial pre-specified a cascade of weight-loss thresholds as confirmatory secondary endpoints, tested in hierarchical order to control for multiplicity. Each comparison reached statistical significance.

HealthRX.com Response-Tier Framework for STEP-1

The following table reconstructs the responder cascade directly from the published results, presenting each categorical threshold alongside its absolute risk difference and number needed to treat (NNT). This framing helps clinicians contextualize what "response" means across different clinical goals.

An NNT of 2 for ≥10% weight loss is striking. For comparison, orlistat trials typically report NNTs of 5 to 8 for achieving ≥5% loss. The 32% rate at the ≥20% threshold placed semaglutide in territory previously occupied only by bariatric surgery procedures like sleeve gastrectomy and Roux-en-Y gastric bypass, which produce 20 to 30% total body weight loss in most long-term registries.

Absolute Weight Loss and Body Composition

Mean baseline body weight was approximately 105 kg in both groups. The −14.9% translates to roughly 15.3 kg (33.7 lb) of absolute weight loss in the semaglutide arm. Placebo participants lost about 2.6 kg. While the trial did not include DEXA body-composition substudies, waist circumference served as a proxy for visceral adiposity. Semaglutide reduced waist circumference by −13.54 cm versus −4.13 cm on placebo (estimated treatment difference: −9.42 cm; 95% CI, −10.30 to −8.53).

This magnitude of waist-circumference reduction is clinically relevant. A reduction of roughly 10 cm corresponds to meaningful decreases in visceral fat volume on CT imaging, based on cross-sectional data from other cohorts.

Week-by-Week Trajectory: When Does the Curve Flatten?

The published weight-loss curves show a consistent pattern. Weight loss in the semaglutide group was steepest between weeks 4 and 28, corresponding to both the dose-escalation phase (completed by week 16) and the initial months of full-dose exposure. The rate of loss decelerated progressively between weeks 28 and 52, and the curve approached a plateau between weeks 52 and 68.

By week 20 (roughly the first assessment point after reaching the full 2.4 mg dose for four weeks), participants had already lost approximately 10% of body weight. The remaining 5 percentage points accrued over the subsequent 48 weeks. This pattern has practical implications for clinical management:

• If a patient has not lost at least 5% by week 20, the likelihood of achieving a strong response by week 68 is low. This aligns with the FDA-approved Wegovy label, which recommends reassessing therapy if a patient has not lost ≥5% at 16 weeks on the maintenance dose.
• The plateau phase does not represent treatment failure. It reflects a new energy-balance equilibrium. Discontinuation studies (STEP-4 and the STEP-1 extension) confirmed that weight regain begins within weeks of stopping semaglutide.

The placebo group's weight-loss curve was essentially flat after the first 12 weeks, averaging about −2% through the remainder of the trial. The lifestyle intervention alone produced minimal sustained effect, consistent with decades of behavioral-intervention data.

Cardiometabolic Secondary Endpoints

STEP-1 was not powered for cardiovascular outcomes (that was SELECT's job), but it captured metabolic biomarkers as secondary endpoints. All changes favored semaglutide.

The CRP reduction is noteworthy. C-reactive protein dropped by over 50% from baseline in the semaglutide arm. Whether this reflects direct anti-inflammatory effects of GLP-1 receptor agonism or is simply a downstream consequence of fat-mass reduction remains debated. The SELECT trial later showed cardiovascular event reduction with semaglutide 2.4 mg, and post-hoc mediation analyses suggested CRP reduction was one contributor to the benefit.

Subgroup Consistency

The trial reported forest plots for the primary endpoint across pre-specified subgroups: sex, age (<65 vs ≥65), race, ethnicity, baseline BMI (<35 vs ≥35 vs ≥40 kg/m²), and prediabetes status. The treatment effect was consistent across all subgroups, with no statistically significant interactions.

A few patterns worth noting, even without formal interaction significance:

• Participants with BMI ≥40 at baseline lost a numerically larger absolute amount of weight but a similar percentage.
• Older adults (≥65 years) showed slightly smaller percentage weight loss, a pattern seen across all anti-obesity medication trials and likely reflecting lower baseline caloric intake and muscle-mass differences.
• Participants with prediabetes at baseline showed comparable weight loss to those without, but their glycemic improvements were more pronounced.

Limitations the Authors Acknowledged

The STEP-1 investigators listed several limitations directly in the publication. First, the trial enrolled a predominantly White population (75%), with smaller representation of Black (6%), Asian (5%), and other racial groups. Generalizability to other populations requires caution, though the STEP-2 and STEP-5 trials (which included more diverse cohorts and those with type 2 diabetes) showed broadly similar efficacy signals.

Second, the trial excluded individuals with type 2 diabetes. This was intentional, as the STEP-2 trial ran in parallel for that population, but it means the −14.9% figure should not be directly extrapolated to patients with diabetes, who consistently lose less weight on GLP-1 receptor agonists.

Third, the 68-week duration, while longer than many obesity trials, does not address durability beyond that window. The STEP-5 trial later extended to 104 weeks and showed maintained weight loss of approximately 15.2% in the semaglutide group, suggesting the plateau is durable as long as treatment continues.

Fourth, the lifestyle intervention (counseling every four weeks) is more intensive than typical real-world clinical practice. Real-world data from the STEP-1 extension and insurance-claims analyses have generally shown slightly smaller average weight loss (10 to 13%), which reflects variable adherence, missed doses, and less structured dietary support.

What Happened After STEP-1

STEP-1 was the registration trial that led to the FDA approval of Wegovy (semaglutide 2.4 mg) in June 2021. The STEP program as a whole included eight trials. STEP-3 added intensive behavioral therapy. STEP-4 tested a withdrawal design. STEP-5 extended follow-up. Each confirmed the central finding: semaglutide 2.4 mg produces roughly 15% weight loss that persists on treatment and reverses off treatment.

The SELECT trial (2023) subsequently demonstrated a 20% reduction in major adverse cardiovascular events with semaglutide 2.4 mg in adults with obesity and established cardiovascular disease but without diabetes, earning Wegovy a cardiovascular risk-reduction indication.

Frequently asked questions

›What was the exact primary endpoint result in STEP-1?

Mean percent body weight change at week 68 was −14.9% with semaglutide 2.4 mg versus −2.4% with placebo. The estimated treatment difference was −12.4 percentage points (95% CI, −13.4 to −11.5; P <0.001).

›What percentage of participants lost 20% or more body weight?

32.0% of semaglutide-treated participants achieved ≥20% weight loss at week 68, compared with 1.7% on placebo.

›How quickly did weight loss occur in STEP-1?

Weight loss was steepest between weeks 4 and 28. By week 20, participants had lost approximately 10% of body weight. The remaining weight loss accrued more gradually over the next 48 weeks.

›Did STEP-1 include patients with type 2 diabetes?

No. STEP-1 excluded participants with type 2 diabetes. The parallel STEP-2 trial enrolled that population and found somewhat smaller weight loss (approximately 9.6% with semaglutide 2.4 mg).

›Was the weight loss sustained after week 68?

The STEP-5 extension trial demonstrated that weight loss was maintained at approximately 15.2% through 104 weeks of continuous semaglutide treatment. Discontinuation studies (STEP-4) showed weight regain begins within weeks of stopping the drug.

›How diverse was the STEP-1 study population?

The trial was predominantly White (75.1%), with 6% Black participants and 5% Asian participants. Later trials in the STEP program enrolled more diverse cohorts.

›What cardiometabolic improvements did semaglutide produce in STEP-1?

Semaglutide reduced systolic blood pressure by about 6 mmHg, CRP by over 50%, triglycerides by roughly 18%, and fasting glucose by about 8 mg/dL, all versus placebo. These were secondary endpoints.

›How does the 14.9% weight loss compare to bariatric surgery?

Sleeve gastrectomy and Roux-en-Y gastric bypass typically produce 20 to 30% total body weight loss. Semaglutide's 14.9% average is lower, but the 32% of participants reaching ≥20% loss overlaps with the lower end of surgical outcomes.

›What does the Wegovy label say about stopping treatment for non-response?

The FDA-approved label recommends discontinuing Wegovy if a patient has not achieved at least 5% weight loss after 16 weeks on the full maintenance dose (2.4 mg).

›Did baseline BMI affect the treatment response?

Subgroup analyses showed consistent relative efficacy across BMI strata (<35, 35 to <40, and ≥40 kg/m²). Those with higher baseline BMI lost more absolute weight but a similar percentage.

References

1. Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med. 2021;384(11):989-1002. PubMed
2. FDA. Wegovy (semaglutide) injection prescribing information. June 2021. FDA Label
3. Garvey WT, Batterham RL, Bhatt DL, et al. Two-year effects of semaglutide in adults with overweight or obesity: the STEP 5 trial. Nat Med. 2022;28(10):2083-2091. PubMed
4. Arterburn DE, Olsen MK, Smith VA, et al. Association between bariatric surgery and long-term survival. JAMA. 2015;313(1):62-70. PubMed
5. Lincoff AM, Brown-Frandsen K, Colhoun HM, et al. Semaglutide and cardiovascular outcomes in obesity without diabetes. N Engl J Med. 2023;389(24):2221-2232. PubMed
6. Jensen MD, Ryan DH, Apovian CM, et al. 2013 AHA/ACC/TOS guideline for the management of overweight and obesity in adults. Circulation. 2014;129(25 Suppl 2):S102-S138. PubMed