As of 2026, the toxicological profiles of 4-substituted tryptamines ? 4-Pro-MET, 4-HO-MET, 4-AcO-MET, and 4-AcO-DMT ? are barely characterized. No LD50 values exist. No long-term toxicity studies, either. That's the blunt reality for anyone trying to assess the safety of this compound class. What follows is an honest look at what we actually know, what we're borrowing from structural analogues, and where the gaps are still wide open.

Very little. For 4-Pro-MET specifically, there are zero published toxicological studies ? the compound only surfaced as a research chemical in August/September 2025. For the broader class of 4-substituted tryptamines, here's what we can actually point to:

• 4-HO-MET: One published non-fatal intoxication case (Journal of Analytical Toxicology, 2021) with a plasma concentration of 193 ng/mL. An in vitro metabolic study identified 12 metabolites via human liver microsomes (Forensic Science International, 2018).
• 4-PrO-DMT: Mouse behavioral data from Glatfelter et al. (2023) ? head-twitch response ED50 of 0.31 mg/kg, hypothermia potency at 11.7 mg/kg, and hypolocomotion potency at 4.8 mg/kg. No mortality data.
• Psilocybin/Psilocin: By far the best-studied 4-substituted tryptamines. Psilocybin's LD50 in rats sits at approximately 280 mg/kg (oral), giving a therapeutic ratio of roughly 1000x relative to active doses. No confirmed human fatalities from psilocybin alone appear in the medical literature.

Without LD50 data for 4-Pro-MET and 4-HO-MET, the lethal dose threshold is genuinely unknown. Psilocybin's high therapeutic ratio hints that 4-substituted tryptamines may share wide safety margins. But hints aren't data ? and this can't be confirmed without direct testing.

5-HT2B Cardiac Risk

This is the concern that keeps coming up. Binding data from Glatfelter et al. (2023) show 4-PrO-DMT has its highest affinity at 5-HT2B (Ki = 17 nM). Chronic stimulation of 5-HT2B receptors on cardiac valve interstitial cells drives fibroblast proliferation and valve thickening ? the same mechanism that sank fenfluramine ("fen-phen") and restricted ergot-derived medications. But there's a key distinction: fenfluramine was taken daily for months to years. Tryptamine use is typically intermittent. Whether intermittent 5-HT2B agonism carries real cardiac risk? Still an open question.

Hepatotoxicity

Both 4-Pro-MET and 4-AcO-MET are ester prodrugs that undergo first-pass hepatic processing. No hepatotoxicity data exist for either compound. The metabolic byproducts ? propanoic acid from 4-Pro-MET, acetic acid from 4-AcO-MET ? are naturally occurring and non-toxic at doses this small. That said, we can't rule out reactive metabolites or idiosyncratic liver reactions without formal safety testing.

Serotonin Syndrome Risk

Serotonin syndrome from a single tryptamine at normal doses? Extremely unlikely. Combine it with other serotonergic agents, though, and the risk jumps (see C19 on interactions). The symptoms ? hyperthermia, muscle rigidity, autonomic instability ? are genuine medical emergencies requiring immediate intervention.