Tirzepatide vs semaglutide: JASSO subgroup

A subpopulation analysis of the SURMOUNT-5 head-to-head trial, published in Current Medical Research and Opinion on 11 July 2026, applied Japan Society for the Study of Obesity (JASSO) criteria to identify which participants qualified as having clinical obesity disease^[1]. Of the 750 adults enrolled in SURMOUNT-5, 383 (51%) met those criteria. Among that group, tirzepatide produced a mean body-weight reduction of 20.1% at 72 weeks versus 12.9% on semaglutide, a 7.2-percentage-point difference (95% CI: 9.3 to 5.1; p less than 0.001)^[1].

What the JASSO criteria select

Japan's national health insurance system covers pharmacotherapy for obesity disease rather than obesity by BMI alone. The JASSO criteria require one of two conditions: a BMI between 27 and 35 kg/m2 with hypertension or dyslipidaemia plus two or more obesity-related health disorders, or a BMI of 35 or above with at least one such disorder^[1]. Applying that filter to the global SURMOUNT-5 cohort let Kitamoto and colleagues test whether the efficacy picture from the overall trial held inside the patient population most relevant to Japanese clinical practice.

It did. The 383 participants meeting JASSO criteria produced results that tracked closely with the full SURMOUNT-5 dataset. The overall trial, published in the New England Journal of Medicine in July 2025, reported tirzepatide at 20.2% mean weight loss versus 13.7% on semaglutide over 72 weeks in 751 adults with obesity but without type 2 diabetes.

The numbers in the JASSO subgroup

In the 190 participants on tirzepatide who met JASSO criteria, mean body-weight reduction reached 20.1% at week 72. In the 193 on semaglutide, it reached 12.9%. The 7.2-percentage-point advantage for tirzepatide was statistically significant^[1]. Proportional responder analyses told the same story: tirzepatide participants more frequently hit every threshold tested, from 10% weight loss up through 30%. Waist circumference reductions favoured tirzepatide beginning at week 4, and BMI reductions favoured tirzepatide from week 12 onward.

The safety profile in the JASSO subgroup was consistent with the main SURMOUNT-5 result. Gastrointestinal events, nausea, diarrhoea, vomiting, and constipation, were the most common adverse effects in both groups, dose-dependent and consistent with the GLP-1 receptor agonist class profile.

What this analysis does and does not settle

The analysis is a post-hoc subgroup extraction from SURMOUNT-5, not a prospectively powered trial in Japanese patients. The authors were transparent about that limit. SURMOUNT-5 was a global trial^[2]; the JASSO subgroup was selected retrospectively by applying published insurance-eligibility criteria to baseline data. No randomisation was stratified by JASSO status, so imbalances between arms are possible. And like the parent trial, this analysis excluded adults with type 2 diabetes, meaning the findings apply to obesity without established diabetes.

The finding is still clinically useful. Japan currently lacks a prospective head-to-head trial of tirzepatide and semaglutide conducted in a Japanese obesity-disease population under domestic treatment conditions. Until one exists, this subgroup analysis provides the best available controlled comparison for the patient group Japanese clinicians are insuring.

Where this fits in the wider trial picture

SURMOUNT-5 as a whole established a 6.5-percentage-point weight-loss advantage for tirzepatide over semaglutide in adults with obesity without diabetes^[2]. The JASSO subgroup adds one more data point: the advantage holds, and appears numerically similar, in the patient population defined by Japan's insurance criteria for obesity pharmacotherapy. That consistency across analytical cuts matters for clinicians and payers deciding between the two agents. The tirzepatide and semaglutide pages on this site carry the full trial evidence base, regulatory status, and safety profiles behind both drugs.

Clinical implications for prescribers

Both tirzepatide and semaglutide are prescription-only agents indicated for chronic weight management. In Japan, prescribing for obesity disease under national insurance requires patients to meet JASSO thresholds, which is the exact population this subgroup captures. Clinicians choosing between the two agents in that setting now have controlled comparison data drawn from the same patients who would qualify for reimbursed treatment. The magnitude of the difference, roughly 7 percentage points at 72 weeks, is consistent across both the JASSO subgroup and the full SURMOUNT-5 cohort, which strengthens confidence that the overall trial result applies to the reimbursement-eligible population specifically.

Adverse-event profiles matter as much as efficacy when choosing between similarly classed agents for chronic use. In the JASSO subgroup, the safety signal did not diverge meaningfully from the parent trial. Gastrointestinal events were the dominant tolerability concern in both arms, and discontinuation rates were consistent with the broader programme. Neither drug introduced a new safety signal in this extract of the SURMOUNT-5 population.

What we do not yet know

The subgroup analysis covers 72-week weight-loss endpoints. It does not report body composition data, metabolic markers beyond weight and waist circumference, or long-term cardiovascular outcomes. The authors note that a dedicated prospective study in Japanese patients with obesity disease would be needed to confirm the findings. Japan-specific regulatory and reimbursement decisions for tirzepatide continue to evolve and are not settled by a retrospective subgroup. The 72-week timeframe also leaves open the question of durability: what weight trajectories look like in the JASSO population at year two or beyond is not answered by this paper.