STEP-8 Trial: A Plain-English Overview of What It Established

# STEP-8 Trial: A Plain-English Overview of What It Established

<TLDR>
STEP-8 was the first randomized trial to pit semaglutide 2.4 mg (once weekly) directly against liraglutide 3.0 mg (once daily) for weight management. After 68 weeks, people on semaglutide lost roughly 15.8% of their body weight compared with 6.4% on liraglutide, a difference large enough to reshape prescribing decisions across obesity medicine.
</TLDR>

<AtAGlance>
| Detail | Value |
|---|---|
| **Trial name** | STEP-8 (Semaglutide Treatment Effect in People with Obesity 8) |
| **N** | 338 (randomized) |
| **Intervention** | Subcutaneous semaglutide 2.4 mg once weekly |
| **Active comparator** | Subcutaneous liraglutide 3.0 mg once daily |
| **Additional arm** | Matched placebo injections for both drugs (double-dummy design) |
| **Duration** | 68 weeks (16-week dose-escalation + 52-week maintenance) |
| **Primary endpoint** | Percentage change in body weight from baseline to week 68 |
| **Key result** | −15.8% (semaglutide) vs −6.4% (liraglutide); estimated treatment difference −9.4 percentage points (p &lt;0.001) |
| **Publication** | [Rubino et al., JAMA 2022](https://pubmed.ncbi.nlm.nih.gov/35015037/) |
</AtAGlance>

{/* HRX:framework */}

## Why This Trial Exists

Before STEP-8, clinicians choosing between semaglutide and liraglutide for obesity had to rely on cross-trial comparisons. The STEP 1 program showed [semaglutide 2.4 mg producing about 14.9% weight loss](https://pubmed.ncbi.nlm.nih.gov/33567185/) at 68 weeks, while the SCALE Obesity trial reported [roughly 8% loss with liraglutide 3.0 mg](https://pubmed.ncbi.nlm.nih.gov/25673378/) at 56 weeks. Those numbers came from different patient populations, different background diets, and different statistical methods. STEP-8 was designed to remove the guesswork by putting both drugs in the same trial, with the same patients, under the same conditions.

The sponsor (Novo Nordisk) funded the study and registered it as an open-label trial with double-dummy placebo arms, a design choice worth examining closely.

## Who Was Enrolled

Participants were adults aged 18 or older with a BMI of 30 kg/m² or higher, or 27 kg/m² or higher with at least one weight-related comorbidity (hypertension, dyslipidemia, obstructive sleep apnea, or cardiovascular disease). All were recruited from 19 U.S. sites.

Key exclusion criteria: diabetes (type 1 or type 2), prior bariatric surgery, a history of pancreatitis, or current use of medications known to affect weight. Participants with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia syndrome type 2 were also excluded, consistent with the [FDA prescribing information for both agents](https://www.accessdata.fda.gov/drugsatfda_docs/label/2021/215256s000lbl.pdf).

The mean baseline BMI was about 37.5 kg/m², and roughly 78% of participants were women. The population skewed white (about 72%), which limits how confidently results generalize to other demographic groups.

## Study Design: The Double-Dummy Trick

STEP-8 used a 2:1:1:1 randomization scheme. For every five participants enrolled, two received active semaglutide plus a daily placebo injection mimicking liraglutide. One received active liraglutide plus a weekly placebo injection mimicking semaglutide. One received weekly placebo alone, and one received daily placebo alone.

This double-dummy approach means each participant gave themselves both a daily and a weekly injection throughout the 68-week trial. Nobody, including the participants or the site staff measuring outcomes, could determine group assignment from injection frequency alone.

Semaglutide was titrated from 0.25 mg weekly up to 2.4 mg over 16 weeks. Liraglutide was titrated from 0.6 mg daily up to 3.0 mg over 4 weeks. Both dose-escalation schedules matched the [FDA-approved titration protocols](https://www.accessdata.fda.gov/drugsatfda_docs/label/2021/215256s000lbl.pdf). All participants also received standardized lifestyle counseling: a 500 kcal/day deficit diet and 150 minutes of physical activity per week.

The trial was open-label at the sponsor level (Novo Nordisk knew which drug was which), but participants and investigators were blinded. This is a common compromise in industry trials where the same company manufactures both drugs.

## What Was Measured

**Primary endpoint:** Percentage change in body weight from baseline to week 68.

**Confirmatory secondary endpoints (tested in hierarchical order):**
- Proportion of participants achieving ≥10% weight loss
- Proportion achieving ≥15% weight loss
- Proportion achieving ≥20% weight loss
- Change in waist circumference

The hierarchical testing procedure is important. Each secondary endpoint was only considered statistically significant if all prior endpoints in the sequence also cleared the significance threshold. This controls for the multiple-comparison problem that plagues trials with many endpoints.

## The Results, in Detail

### Body Weight Loss

The primary outcome told a clear story. At 68 weeks, semaglutide-treated participants lost a mean of 15.8% of their body weight. Those on liraglutide lost 6.4%. The two placebo groups lost 1.9% (weekly placebo) and 1.7% (daily placebo), respectively.

The estimated treatment difference between semaglutide and liraglutide was [−9.4 percentage points (95% CI: −12.0 to −6.8; p &lt;0.001)](https://pubmed.ncbi.nlm.nih.gov/35015037/).

| Outcome | Semaglutide 2.4 mg | Liraglutide 3.0 mg | Difference |
|---|---|---|---|
| Mean % weight loss | −15.8% | −6.4% | −9.4 pp |
| ≥5% weight loss | 87.2% | 58.1% | +29.1 pp |
| ≥10% weight loss | 70.9% | 25.6% | +45.3 pp |
| ≥15% weight loss | 55.6% | 12.0% | +43.6 pp |
| ≥20% weight loss | 38.5% | 6.0% | +32.5 pp |

Those categorical thresholds matter because the FDA considers ≥5% body weight loss clinically meaningful for obesity pharmacotherapy. Nearly 9 in 10 semaglutide participants hit that bar. Roughly 7 in 10 lost at least 10%, a threshold associated with improvement in obstructive sleep apnea, NAFLD, and glycemic control in pre-diabetes.

### Waist Circumference and Cardiometabolic Markers

Waist circumference dropped by 13.54 cm with semaglutide versus 6.69 cm with liraglutide, a difference of −6.85 cm (95% CI: −9.17 to −4.53).

Improvements in systolic blood pressure, C-reactive protein, and lipid panels all favored semaglutide, though most of these were exploratory endpoints and not powered for formal hypothesis testing.

### The Time Course

Weight loss with semaglutide continued to diverge from liraglutide throughout the trial. By week 20 (shortly after full-dose semaglutide was reached), the semaglutide group had already lost about 10% body weight vs. roughly 5% with liraglutide. The curves kept separating through week 68, suggesting the plateau had not yet been reached for semaglutide at trial's end.

## Safety and Tolerability

Gastrointestinal (GI) side effects were the most common adverse events in both groups, consistent with the known [GLP-1 receptor agonist class profile](https://www.accessdata.fda.gov/drugsatfda_docs/label/2021/215256s000lbl.pdf).

| Adverse Event | Semaglutide (%) | Liraglutide (%) |
|---|---|---|
| Nausea | 43.9 | 38.2 |
| Diarrhea | 29.7 | 20.0 |
| Constipation | 25.7 | 16.4 |
| Vomiting | 20.9 | 12.7 |

Discontinuation due to adverse events was 3.4% for semaglutide and 12.7% for liraglutide. That gap is striking: despite higher rates of nausea and vomiting, fewer semaglutide participants actually quit. The likely explanation is that semaglutide's more dramatic weight-loss results motivated participants to push through early GI discomfort. Liraglutide's slower titration (4 weeks to full dose vs. 16 weeks for semaglutide) may also have affected early tolerability perceptions differently.

No deaths occurred. Serious adverse events were infrequent and balanced across groups. Gallbladder-related events (cholelithiasis) occurred in a small number of participants in both active arms, a known risk with rapid weight loss of any cause.

## Limitations the Authors Acknowledged

The [published paper](https://pubmed.ncbi.nlm.nih.gov/35015037/) lists several limitations worth taking seriously:

1. **No diabetes subgroup.** Patients with type 2 diabetes were excluded, so these results cannot be directly applied to that population. STEP-2 and the SCALE Diabetes trial cover those patients separately, with different effect sizes.

2. **Short duration.** At 68 weeks, the trial cannot answer questions about long-term weight maintenance, cardiovascular event reduction, or durability of metabolic improvements. The SELECT trial (published in 2023) later addressed the cardiovascular question for semaglutide specifically.

3. **Predominantly white, female, U.S.-based population.** Generalizability to men, non-white racial groups, and patients outside the United States is uncertain.

4. **Industry sponsorship.** Novo Nordisk funded the trial, manufactures both drugs, and employed several authors. The double-dummy blinding mitigates some bias, but the sponsor's involvement in data analysis and manuscript preparation is a standard concern.

5. **No tirzepatide comparator.** STEP-8 was designed before tirzepatide's obesity data were available. A semaglutide-vs-tirzepatide comparison would require a separate head-to-head trial (no such study has been published as of mid-2025).

6. **Route of administration difference at baseline.** Semaglutide is given weekly; liraglutide is given daily. Even with double-dummy blinding, the psychological experience of a once-weekly "real" injection may differ from a daily one. This is difficult to control for in any trial design.

## What STEP-8 Changed in Practice

Before this trial, clinicians could reasonably argue that semaglutide and liraglutide were "in the same class" and roughly interchangeable for weight management. STEP-8 ended that argument. A 9.4-percentage-point difference in weight loss is not subtle. It is large enough that major obesity-medicine guidelines, including the [2022 American Gastroenterological Association guideline on pharmacological management of obesity](https://pubmed.ncbi.nlm.nih.gov/36273831/), specifically reference the superiority data when positioning semaglutide as a preferred GLP-1 option.

From a patient perspective, STEP-8 also clarified the injection-burden trade-off. Semaglutide requires one injection per week. Liraglutide requires one per day. The drug that works better also requires less frequent dosing, which simplifies the treatment decision for most patients.

Insurance formulary placement has been affected as well. Several major payers revised their step-therapy requirements after STEP-8, no longer mandating a trial of liraglutide before approving semaglutide. The cost difference remains significant (both drugs carry list prices above $1,000/month in the U.S.), but the efficacy gap has shifted the conversation from "which is covered" toward "which produces meaningful results."

## The Bigger Picture

STEP-8 sits within a larger body of evidence. The STEP program includes trials numbered 1 through 17 (and counting), each examining semaglutide 2.4 mg in a different clinical scenario. STEP-8 is unique because it is the only one designed as a head-to-head comparison against another GLP-1 receptor agonist.

For patients and clinicians weighing their options, STEP-8 provides some of the most directly useful data available: if you are choosing between these two specific drugs for weight management in a person without diabetes, semaglutide produces roughly 2.5 times the weight loss, with a lower discontinuation rate, at one-seventh the injection frequency.

<FAQAccordion>
<FAQ question="What was the primary finding of the STEP-8 trial?">
Semaglutide 2.4 mg (once weekly) produced 15.8% body weight loss at 68 weeks, compared with 6.4% for liraglutide 3.0 mg (once daily). The 9.4-percentage-point difference was statistically significant (p &lt;0.001). This was the [first direct head-to-head comparison](https://pubmed.ncbi.nlm.nih.gov/35015037/) of these two GLP-1 receptor agonists for obesity treatment.
</FAQ>

<FAQ question="How many participants were in the STEP-8 trial?">
The trial enrolled 338 participants across 19 sites in the United States. Randomization followed a 2:1:1:1 scheme, with the largest group assigned to semaglutide. About 78% of participants were women, and the mean BMI at baseline was approximately 37.5 kg/m².
</FAQ>

<FAQ question="Were people with type 2 diabetes included in STEP-8?">
No. The trial excluded anyone with type 1 or type 2 diabetes. Results from STEP-8 apply specifically to adults with obesity or overweight (with comorbidities) who do not have diabetes. Separate trials (STEP-2 for semaglutide, SCALE Diabetes for liraglutide) addressed diabetic populations.
</FAQ>

<FAQ question="What were the most common side effects in STEP-8?">
Gastrointestinal symptoms dominated both groups. Nausea affected about 44% of semaglutide users and 38% of liraglutide users. Diarrhea, constipation, and vomiting were also common. Despite higher GI event rates, the semaglutide group had a lower dropout rate (3.4%) than the liraglutide group (12.7%).
</FAQ>

<FAQ question="How does semaglutide dosing compare to liraglutide dosing for obesity?">
Semaglutide 2.4 mg is injected subcutaneously once per week, titrated up over 16 weeks. Liraglutide 3.0 mg is injected subcutaneously once per day, titrated up over 4 weeks. The [FDA-approved labels](https://www.accessdata.fda.gov/drugsatfda_docs/label/2021/215256s000lbl.pdf) for both drugs specify these schedules. STEP-8 used double-dummy placebo injections so participants in all arms gave themselves both daily and weekly shots.
</FAQ>

<FAQ question="Did STEP-8 look at cardiovascular outcomes?">
No. STEP-8 was a 68-week weight-loss trial, not a cardiovascular outcomes trial. Changes in blood pressure, lipids, and C-reactive protein were measured as secondary or exploratory endpoints, but the trial was not designed or powered to detect differences in heart attacks, strokes, or cardiovascular death. The SELECT trial (2023) later demonstrated cardiovascular benefits for semaglutide 2.4 mg in a separate, larger population.
</FAQ>

<FAQ question="How was the STEP-8 trial blinded?">
The study used a double-dummy design. Every participant received both a weekly injection and a daily injection. Depending on group assignment, one of those injections contained active drug and the other was a matched placebo. This prevented participants and investigators from guessing assignments based on dosing frequency.
</FAQ>

<FAQ question="Is STEP-8 data applicable to oral semaglutide?">
Not directly. STEP-8 tested subcutaneous semaglutide 2.4 mg (branded as Wegovy for obesity). Oral semaglutide (Rybelsus) is approved for type 2 diabetes at doses up to 14 mg and has separate efficacy data. Higher oral doses (up to 50 mg) are under investigation for obesity but were not part of the STEP-8 protocol.
</FAQ>

<FAQ question="Why did fewer people drop out of the semaglutide group despite more nausea?">
The most likely explanation is that greater weight loss motivated continued use. When patients see dramatic results on the scale, they tend to tolerate more GI discomfort. The slower 16-week titration for semaglutide (vs. 4 weeks for liraglutide) may also have spread out early side effects, making them more manageable.
</FAQ>

<FAQ question="Has any trial compared semaglutide to tirzepatide for weight loss?">
As of mid-2025, no published randomized controlled trial has directly compared semaglutide 2.4 mg to tirzepatide for obesity in a head-to-head design. Cross-trial comparisons suggest tirzepatide at its highest approved dose (15 mg) may produce greater weight loss, but only a properly designed head-to-head study can confirm that.
</FAQ>
</FAQAccordion>

<References>
1. Rubino DM, Greenway FL, Khalid U, et al. Effect of weekly subcutaneous semaglutide vs daily liraglutide on body weight in adults with overweight or obesity without diabetes: the STEP 8 randomized clinical trial. *JAMA*. 2022;327(2):138-150. [PubMed](https://pubmed.ncbi.nlm.nih.gov/35015037/)
2. Wilding JPH, Batterham RL, Calanna S, et al. Once-weekly semaglutide in adults with overweight or obesity (STEP 1). *N Engl J Med*. 2021;384(11):989-1002. [PubMed](https://pubmed.ncbi.nlm.nih.gov/33567185/)
3. Pi-Sunyer X, Astrup A, Fujioka K, et al. A randomized, controlled trial of 3.0 mg of liraglutide in weight management (SCALE Obesity and Prediabetes). *N Engl J Med*. 2015;373(1):11-22. [PubMed](https://pubmed.ncbi.nlm.nih.gov/25673378/)
4. FDA. Wegovy (semaglutide) prescribing information. 2021. [FDA Label](https://www.accessdata.fda.gov/drugsatfda_docs/label/2021/215256s000lbl.pdf)
5. Grunvald E, Shah R, Hernaez R, et al. AGA clinical practice guideline on pharmacological interventions for adults with obesity. *Gastroenterology*. 2022;163(5):1198-1225. [PubMed](https://pubmed.ncbi.nlm.nih.gov/36273831/)
</References>