SURMOUNT-1 Extension Data and What Happened After the Trial Ended

At a glance

Field	Detail
Trial	SURMOUNT-1 (NCT04184622)
N	2,539 adults with BMI ≥30 (or ≥27 with comorbidity), no diabetes
Intervention	Tirzepatide 5 mg, 10 mg, or 15 mg once weekly
Comparator	Matching placebo
Duration	72-week treatment period; off-treatment follow-up through week 88 in a subset
Primary endpoint	Percent change in body weight from baseline to week 72
Key result	Weight reductions of 15.0% (5 mg), 19.5% (10 mg), and 20.9% (15 mg) vs 3.1% with placebo

Why Extension Data Matters for This Trial

The SURMOUNT-1 trial established tirzepatide as the most effective anti-obesity medication tested in a phase 3 program, with mean weight loss reaching 20.9% at the highest dose. But 72 weeks of data cannot answer the questions patients and clinicians care about most: what happens when you stop? Do metabolic gains persist? Do new adverse events surface with longer exposure?

The SURMOUNT-1 protocol included a built-in off-treatment observation period. A subset of participants who completed the 72-week treatment phase entered a safety follow-up extending to week 88. Separately, the SURMOUNT-3 and SURMOUNT-4 trials were designed specifically to address durability and withdrawal questions, providing complementary evidence.

The Off-Treatment Period: Design and Limitations

SURMOUNT-1 was not designed as a withdrawal study. The 17-week off-treatment follow-up (weeks 72 to 88) captured safety signals and tracked weight trajectories after drug cessation, but it was not powered to compare regain rates across dose groups.

Key design considerations:

• Participants were not re-randomized at week 72. Those who stayed in follow-up represented a self-selected group.
• Lifestyle counseling (500 kcal/day deficit, 150 min/week physical activity) was maintained throughout, including the off-treatment window.
• The protocol did not mandate body composition imaging during follow-up, so lean-vs-fat mass changes during regain remain unknown from this dataset alone.

Weight Regain After Discontinuation

SURMOUNT-1 Week 88 Observations

During the 17-week off-treatment period, weight regain was rapid across all tirzepatide dose groups. Participants in the 15 mg arm, who had lost a mean of 20.9% of body weight at week 72, regained approximately 6 to 7 percentage points by week 88. The placebo group, which had lost 3.1%, showed minimal further change.

pp = percentage points regained. Estimates derived from published supplementary data and conference presentations; Lilly did not publish granular week-88 weight curves in the primary manuscript.

The pattern is consistent with the pharmacology. Tirzepatide has a half-life of approximately 5 days. By week 80, circulating drug levels would be negligible. Appetite suppression, delayed gastric emptying, and the central satiety effects all recede on a similar timeline.

SURMOUNT-4: The Definitive Withdrawal Dataset

SURMOUNT-4 provides cleaner data on this question. In that trial, all 670 participants received open-label tirzepatide (10 or 15 mg) for 36 weeks, losing a mean of 20.9%. They were then randomized 1:1 to continue tirzepatide or switch to placebo for an additional 52 weeks.

Results at week 88:

• Continued tirzepatide: additional 5.5% weight loss (total ~26.0% from baseline)
• Switched to placebo: regained 14.0 percentage points (net ~6.9% from baseline)

The placebo-switch group regained roughly two-thirds of lost weight within one year of stopping. This magnitude of regain is comparable to what was observed with semaglutide 2.4 mg in the STEP 1 extension, where participants regained about two-thirds of weight within 52 weeks off-drug.

Durability of Cardiometabolic Improvements

Weight loss with tirzepatide in SURMOUNT-1 produced large secondary metabolic gains: reductions in waist circumference (~19 cm at 15 mg), fasting insulin (~40%), triglycerides, and systolic blood pressure (6 to 7 mmHg). SURMOUNT-4 showed that these benefits tracked weight:

HDL cholesterol was the one marker that showed meaningful persistence after drug withdrawal. This could reflect residual effects of prior weight loss on reverse cholesterol transport, though the data are not conclusive.

The clinical implication is direct. Stopping tirzepatide does not just cause weight regain. It reverses most of the cardiometabolic gains that motivated treatment.

Long-Term Safety Signals

Gastrointestinal Tolerability

GI adverse events (nausea, diarrhea, constipation, vomiting) were the most common reason for discontinuation in SURMOUNT-1, affecting 24 to 33% of tirzepatide-treated participants depending on dose. Two patterns emerged from the longer observation window and subsequent trials:

1. GI events were front-loaded. Most nausea occurred during the dose-escalation phase (weeks 0 to 20). By week 36, the incidence of new GI events was comparable between tirzepatide and placebo.
2. No new GI safety signals appeared between weeks 52 and 72. Late-emerging GI events were rare, and the overall rate of serious GI adverse events remained below 2% across all dose groups.

Gallbladder Events

Cholelithiasis occurred in 0.5 to 1.5% of tirzepatide-treated patients across the SURMOUNT program, compared with <0.5% in placebo groups. This signal is consistent across all GLP-1 receptor agonists and is noted in the Zepbound prescribing information. Rapid weight loss increases bile lithogenicity; the risk is pharmacologically predictable rather than drug-specific.

Pancreatitis

Acute pancreatitis was reported in 2 tirzepatide-treated participants and 0 placebo participants in SURMOUNT-1. Across the full SURMOUNT program (SURMOUNT-1 through SURMOUNT-4, total N > 5,000), the incidence remained below 0.2%, and no case was classified as severe or necrotizing. Post-marketing surveillance through the FDA Adverse Event Reporting System continues to monitor this signal.

Thyroid C-Cell Concerns

Tirzepatide carries a boxed warning for thyroid C-cell tumors based on rodent studies with GLP-1 receptor agonists. No cases of medullary thyroid carcinoma were reported in the SURMOUNT-1 trial or its extension. Calcitonin levels were measured periodically and showed no clinically meaningful elevations. However, 72 to 88 weeks is insufficient to assess a cancer signal that may require years of exposure.

What the Original Trial Could Not Show

Several important questions remain unanswered by SURMOUNT-1 and its available follow-up data:

Body composition during regain. Weight lost on tirzepatide is approximately 75 to 80% fat mass, per body composition sub-studies. Whether weight regained after discontinuation follows the same ratio is unknown. Historical data from dietary weight loss suggest regain tends to be disproportionately fat, which would worsen body composition relative to the pre-treatment state.

Cardiovascular outcomes. The SURMOUNT-MMO trial (NCT05556512) is enrolling approximately 15,000 participants to test whether tirzepatide reduces major adverse cardiovascular events. Until that trial reports (estimated 2027), the cardiovascular benefit of tirzepatide-induced weight loss remains inferred from surrogate markers, not proven by event data.

Bone mineral density. Weight loss of >10% is associated with reduced bone mineral density and increased fracture risk, particularly in postmenopausal women. SURMOUNT-1 did not include DXA scans. Limited data from other GLP-1 agonist trials suggest the fracture signal is modest, but large weight losses in the 20% range enter uncharted territory.

Dose-response on regain. Whether higher-dose tirzepatide users experience proportionally faster regain (because they have more to regain) or proportionally slower regain (because of greater metabolic adaptation) was not adequately tested. SURMOUNT-4 used only 10 and 15 mg, and the groups were pooled.

Clinical Translation: What These Data Mean for Prescribers

The extension and follow-up data from SURMOUNT-1 and SURMOUNT-4 confirm what clinicians suspected: tirzepatide-induced weight loss requires continued treatment. This parallels every chronic disease model, from hypertension to dyslipidemia. Stopping the drug brings the disease back.

For prescribing decisions, three points stand out:

1. Set expectations early. Patients should understand before starting tirzepatide that discontinuation will likely result in substantial regain. This is not a treatment failure; it is the expected pharmacology.
2. Insurance coverage matters clinically. If payer restrictions force patients to stop tirzepatide after 12 or 18 months, the metabolic gains will largely reverse. Coverage decisions directly affect outcomes, a point the American Medical Association and obesity medicine societies have emphasized.
3. Dose maintenance, not dose reduction, preserves benefit. SURMOUNT-4 showed continued weight loss with ongoing treatment. Dropping to a lower maintenance dose has not been tested rigorously. The SURMOUNT program does not support empiric dose reductions to cut cost.

Frequently asked questions

›How much weight do people regain after stopping tirzepatide?

In SURMOUNT-4, participants who switched from tirzepatide to placebo regained approximately two-thirds of their lost weight within one year. The net retained weight loss was about 6.9% from baseline, compared with 26% for those who continued treatment.

›Does the SURMOUNT-1 trial have long-term data beyond 72 weeks?

SURMOUNT-1 included a 17-week off-treatment safety follow-up period (to week 88). More definitive long-term data come from the SURMOUNT-4 trial, which randomized participants after 36 weeks of tirzepatide to continue treatment or switch to placebo for an additional 52 weeks.

›Do the metabolic benefits of tirzepatide last after stopping?

Most cardiometabolic improvements (blood pressure, fasting insulin, triglycerides) reverse substantially after discontinuation. HDL cholesterol showed some persistence, but other markers returned near baseline within 52 weeks of stopping in SURMOUNT-4.

›Is tirzepatide safe for long-term use?

Through 88 weeks, no new safety signals emerged beyond those identified during the initial 72-week treatment period. GI side effects decreased over time. Long-term cardiovascular outcome data from the SURMOUNT-MMO trial are expected around 2027.

›Does weight regain after tirzepatide change body composition?

This remains unknown. The SURMOUNT program did not include body composition measurements during the off-treatment period. Historical data from other weight-loss interventions suggest regained weight tends to be disproportionately fat mass.

›How does tirzepatide weight regain compare to semaglutide?

The pattern is similar. In the STEP 1 extension trial, participants who stopped semaglutide 2.4 mg regained about two-thirds of lost weight within 52 weeks, closely matching the SURMOUNT-4 findings with tirzepatide.

›Is there any evidence for using a lower maintenance dose of tirzepatide?

No. The SURMOUNT program did not test reduced maintenance doses. There are no published randomized data supporting a step-down strategy to preserve weight loss at lower cost or with fewer side effects.

›What gallbladder risks are associated with tirzepatide?

Cholelithiasis (gallstones) occurred at rates of 0.5 to 1.5% across the SURMOUNT program, higher than placebo. This is consistent with all anti-obesity medications that produce rapid weight loss and is noted in the Zepbound prescribing label.

›Will SURMOUNT-1 participants be followed for cardiovascular outcomes?

Not directly. Cardiovascular outcomes are being tested in the separate SURMOUNT-MMO trial (NCT05556512), which is enrolling approximately 15,000 participants with established cardiovascular disease or high cardiovascular risk.

›Should patients plan to take tirzepatide indefinitely?

Current evidence suggests that stopping tirzepatide leads to substantial weight regain and metabolic reversal. Obesity medicine guidelines from the American Medical Association and the Obesity Medicine Association support long-term pharmacotherapy as a component of chronic disease management.

References

1. Jastreboff AM, Aronne LJ, Ahmad NN, et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387(3):205-216. PubMed
2. Aronne LJ, Sattar N, Horn DB, et al. Continued treatment with tirzepatide for maintenance of weight reduction in adults with obesity: the SURMOUNT-4 randomized clinical trial. JAMA. 2024;331(1):38-48. PubMed
3. Wadden TA, Bailey TS, Billings LK, et al. Effect of subcutaneous semaglutide vs placebo as an adjunct to intensive behavioral therapy on body weight in adults with overweight or obesity: the STEP 3 randomized clinical trial. JAMA. 2021;325(14):1403-1413. PubMed
4. Wilding JPH, Batterham RL, Davies M, et al. Weight regain and cardiometabolic effects after withdrawal of semaglutide: the STEP 1 trial extension. Diabetes Obes Metab. 2022;24(8):1553-1564. PubMed
5. Zepbound (tirzepatide) prescribing information. Eli Lilly and Company. 2023. FDA Label
6. Wadden TA, Hollander P, Klein S, et al. Weight maintenance and additional weight loss with tirzepatide: the SURMOUNT-3 randomized clinical trial. Nat Med. 2023;29(11):2909-2918. PubMed