5-F-MET, also known as 5-fluoro-MET, GM-2505 or bretisilocin, is a fluorinated tryptamine developed as an investigational 5-HT2A receptor agonist. It has an N-methyl-N-ethyl side chain and a fluorine atom at the 5-position of the indole ring. Unlike most compounds in this group, it has progressed into a published first-in-human Phase I study.
Chemistry and identification
5-F-MET is N-ethyl-2-(5-fluoro-1H-indol-3-yl)-N-methylethanamine. The structure retains the tryptamine scaffold but replaces the hydrogen at the 5-position with fluorine. The development code GM-2505 and the name bretisilocin refer to the same compound in the clinical literature.
It should not be confused with 5-MeO-MET. The two names describe different substitutions: 5-F-MET has a fluorine atom on the ring, while 5-MeO-MET carries a methoxy group.
Pharmacology
Preclinical studies describe GM-2505 as a potent 5-HT2A receptor agonist that also promotes serotonin release and activates 5-HT2C receptors. In the reported assays, it showed antagonist rather than agonist activity at 5-HT2B. Animal studies also found a head-twitch response, supporting psychedelic-like activity in that model (Marek and colleagues, 2025).
First-in-human clinical study
A randomised, double-blind, placebo-controlled Phase I study enrolled 48 healthy volunteers and assessed single intravenous doses of GM-2505. The reported plasma half-life was approximately 40–50 minutes, and the subjective pharmacodynamic effects generally diminished within about two hours. The study found dose-related changes in subjective, neurophysiological and neuroendocrine measures (Marek and colleagues, 2025).
No serious adverse events were reported. Most adverse events were mild, with a smaller proportion classified as moderate. Transient increases in blood pressure and pulse were observed, particularly at higher doses. These results describe a small, closely monitored Phase I study and do not establish safety in patients or the effectiveness of the compound for depression.
Development status and limitations
GM-2505 is being investigated as a potential treatment for major depressive disorder. The published study provides initial human pharmacokinetic and tolerability data, but it was not designed to demonstrate antidepressant efficacy. Larger and later-stage clinical trials would be needed to determine whether the proposed therapeutic effects occur and persist.
The trial findings cannot be equated with recreational accounts of other tryptamines. 5-F-MET has a specific investigational development history and a human Phase I evidence base that most related research chemicals do not have.
Related tryptamines
5-F-MET is related to 4-HO-MET, 5-MeO-MET and 5-MeO-MiPT. Its defining feature is the 5-fluoro substitution on the indole ring.
