SURMOUNT-J reports Japanese obesity results

A phase 3 trial called SURMOUNT-J found that tirzepatide, the drug sold in the United States as Mounjaro and Zepbound, produced large weight losses in Japanese adults with obesity when compared with placebo over 72 weeks ^[1]. The placebo-adjusted change in body weight was -16.1% for the 10 mg dose and -21.1% for the 15 mg dose ^[1].

The study enrolled Japanese adults with what Japan's obesity guidelines call "obesity disease," a category that does not use the same body mass index cutoff as the United States ^[1]. For this trial, participants needed a BMI of at least 27 with two or more obesity-related health problems, or a BMI of at least 35 with one such problem, and people with diabetes were excluded ^[1]. Elsewhere, Japan's broader definition of obesity disease uses a BMI of 25 or higher with excess fat accumulation ^[1]. In the United States, obesity is generally defined starting at a BMI of 30, so patients labeled as having obesity in Japan can have a lower BMI than what triggers an obesity diagnosis here.

Researchers screened 413 people and randomly assigned 267 to receive weekly injections of tirzepatide at 10 mg, tirzepatide at 15 mg, or placebo between May 2021 and June 2023 ^[1]. After one study site was excluded, the analyzed group totaled 225 participants: 73 on the 10 mg dose, 77 on the 15 mg dose, and 75 on placebo ^[1]. The group was 59% men and 41% women, with an average age of about 51 ^[1]. Most participants, 85%, completed both the treatment and the study ^[1].

At week 72, 94% of people on the 10 mg dose and 96% of people on the 15 mg dose had lost at least 5% of their body weight, compared with just 20% of those on placebo ^[1]. The trial also reported improvements in cardiometabolic markers and body composition with tirzepatide, though specific numbers for those measures were not detailed in the published summary ^[1].

Side effects followed a pattern seen in other tirzepatide trials. Treatment-related adverse events occurred in 84% of people on the 10 mg dose and 86% of those on the 15 mg dose, versus 69% on placebo, with gastrointestinal symptoms the most common complaint ^[1]. Few people stopped treatment because of side effects: one person on 10 mg, no one on 15 mg, and three people on placebo discontinued for this reason ^[1]. The trial was funded by Eli Lilly and Company, tirzepatide's manufacturer ^[1].

Why it matters for patients

These results show that tirzepatide's weight-loss effect extends to a Japanese population studied under Japan's own obesity criteria, not just to the Western populations enrolled in earlier SURMOUNT trials ^[1]. The size of the effect, a 16% to 21% placebo-adjusted weight loss depending on dose, lines up with what has been reported in global tirzepatide trials, according to the study authors ^[1].

For patients in the United States, the study is a reminder that obesity definitions and treatment thresholds vary by country. Someone classified as having "obesity disease" in Japan might not meet the BMI threshold used for an obesity diagnosis in the U.S. This does not change how tirzepatide is prescribed in America, but it does mean that international trial data reflects different patient populations than what U.S. doctors typically evaluate. The safety pattern, dominated by gastrointestinal side effects and low discontinuation rates, matches what has already been reported to U.S. patients using Mounjaro or Zepbound ^[1].

What happens next

The findings were published alongside an invited commentary in Lancet Diabetes & Endocrinology, and the trial is registered as NCT04844918 ^[1]. The sources reviewed do not specify whether these Japan-specific results will affect labeling, regulatory review, or prescribing guidance in the United States.