A once-monthly MASH injection cleared its safety bar in a small phase 2 trial

A once-monthly injection for fatty-liver disease, efimosfermin alfa, was generally well tolerated over 24 weeks in a phase 2 trial of 84 patients, its developers reported in The Lancet [s1]. The result is a genuine step for the drug, but a limited one: this trial was designed to test safety and tolerability, not to show that the drug reverses the liver damage it targets [s1].

Efimosfermin (formerly coded BOS-580) is an analogue of fibroblast growth factor 21 (FGF21), a hormone that regulates fat and sugar metabolism and that several companies are trying to turn into a treatment for metabolic dysfunction-associated steatohepatitis (MASH) — the more damaging form of fatty-liver disease, in which fat accumulation drives inflammation and scarring [s1]. Its distinguishing feature is the schedule: a subcutaneous injection every four weeks, against the daily dosing of most drugs in the field [s1].

What the trial did

The trial was a 24-week, randomised, double-blind, placebo-controlled phase 2 study run at 34 sites in the United States [s1]. It enrolled adults aged 18 to 75 with a body-mass index of at least 27 and biopsy-confirmed MASH with moderate or advanced fibrosis — stage F2 or F3 — plus a NAFLD activity score of at least 4 [s1]. Of 1,171 people screened, 84 were randomised 1:1 to efimosfermin 300 mg or placebo, given by subcutaneous injection every four weeks [s1].

Crucially, the primary endpoint was safety and tolerability — treatment-emergent adverse events, changes in blood pressure and heart rate, and grade 3 or 4 laboratory abnormalities at week 24 [s1]. Of those randomised, 48 (57%) had F2 fibrosis and 36 (43%) had F3, and 65 had an evaluable liver biopsy at week 24 [s1].

What it found

Adverse events were reported in 29 of 43 participants (67%) on efimosfermin and 22 of 40 (55%) on placebo, and most were mild or moderate [s1]. The most frequent were gastrointestinal effects, which were transient and clustered in the first weeks of treatment [s1]. There were no clinically meaningful changes in vital signs, no clinically significant grade 3 or higher laboratory abnormalities, and no deaths or adverse events worse than grade 3 during the study [s1]. The trial's authors concluded the drug was generally well tolerated and that the results support its further development [s1].

How to read it

The honest reading is that this is a safety trial that passed a safety test. The endpoint it was built to answer — is a monthly FGF21 analogue tolerable in people with moderate-to-advanced MASH fibrosis — it answered in the affirmative, over 24 weeks, in a small sample [s1]. What it does not establish is the thing patients and clinicians actually need to know: whether the drug resolves steatohepatitis or reverses fibrosis. Those biopsy-based efficacy measures were not the primary endpoint, and a 24-week study in 84 people, with 65 evaluable biopsies, is not powered to settle them [s1].

The scale is the other caveat. Eighty-four randomised participants is a phase 2 signal-finding size, and a favourable tolerability profile at that scale can shift when a drug is tested in the hundreds or thousands of patients that a phase 3 fibrosis trial requires [s1]. A linked Lancet commentary frames the monthly-dosing convenience as the drug's main point of differentiation while noting that its place depends on efficacy data still to come [s2]. The trial also carries a published correction, a reminder that even the reported figures are the record as amended [s1].

Why it matters

MASH has only recently gained its first approved drug, and the field is crowded with candidates working through different mechanisms — FGF21 analogues, thyroid-hormone-receptor agonists, and the incretin drugs already used for weight loss. A well-tolerated monthly injection would be a practical advantage if — and only if — the efficacy data follow. This trial keeps efimosfermin in the running; it does not move it to the front.

Readers can set it against other approaches to the same disease covered here: the phase 3 SYNCHRONIZE trial of survodutide in MASLD, the European trial of dapagliflozin against a resmetirom backdrop, and the broader question of how common fatty-liver disease is and what actually reverses it.

What to watch

The decisive evidence is the next tier of trials: larger, longer studies with biopsy-confirmed resolution of steatohepatitis or improvement in fibrosis as their primary endpoints [s1][s2]. Until those report, efimosfermin's monthly schedule is a promising feature attached to an unproven benefit, and the safety result reported here should be read as a green light to keep testing, not as evidence that the drug works.

This article describes early-stage clinical-trial research and drug safety and is not medical advice. Decisions about liver-disease treatment are for patients and their treating clinicians.

Sources

• Efimosfermin alfa (BOS-580) once per month in people with MASH with F2 or F3 fibrosis: a 24-week, randomised, double-blind, placebo-controlled, phase 2 trial — The Lancet, online 5 February 2026
• Once-monthly efimosfermin for non-cirrhotic MASH — The Lancet, 5 February 2026 (linked commentary)