5-MeO-αMT, also known as 5-methoxy-α-methyltryptamine or as α,O-dimethylserotonin (α,O-DMS or Alpha-O), is a psychedelic drug of the tryptamine, α-alkyltryptamine, and 5-methoxytryptamine families. It is a derivative of α-methyltryptamine (αMT) and an analogue of 5-MeO-DMT. The drug is said to be the most potent psychedelic of the simple indolealkylamines (i.e., tryptamines). It is taken orally and is used at doses of 2 to 4 mg.

In his book TiHKAL (Tryptamines I Have Known and Loved), Alexander Shulgin lists the dose range of 5-MeO-AMT as 2.5 to 4.5 mg and its duration as 12 to 18 hours. However, a wider dose range of 0.5 to 15 mg has also been reported.

5-MeO-AMT has supposedly been sold as 4 mg tablets by the street name Alpha-O and taken as a recreational drug. Since the DEA arrests of the makers of a huge percentage of the United States' LSD in 2000, 5-MeO-AMT may have occasionally been sold under the guise of LSD in liquid, sugar cube, or blotter form, though this may be due to DEA reports of finding it on sugar cubes and blotters like LSD.

Shulgin has described the effects of 5-MeO-AMT in TiHKAL.

If misrepresented as LSD, 5-MeO-AMT can be extremely dangerous; users may take a number of "hits" of 5-MeO-AMT, assuming that it is LSD. Unlike LSD, which is very safe in overdose, 5-MeO-AMT can be very harmful or fatal. Particularly sensitive individuals can experience symptoms of overdose at dosages in the normal (for most users) range — as low as 20 mg. This has led to at least a few hospitalizations and possibly more than one death. It is likely that the overdose potential of the compound is due to its sympathomimetic effects, as the side effects noted in overdose cases include cardiac arrhythmia and seizure. It also seems that oral consumption is safer than insufflation.^{[citation needed]}

Gloria Discerni, 18, died after overdosing on a drug initially believed to be LSD. Authorities learned months later that the drug wasn't LSD but a "designer drug" identified as 5-MeO-AMT.

5-MeO-AMT acts as a non-selective serotonin receptor agonist, including of the serotonin 5-HT_1A, 5-HT_2A, and 5-HT_2B receptors, among others. Its EC₅₀Tooltip half-maximal effective concentration at the serotonin 5-HT_2A receptor has been found to be 2 to 8.4 nM. In relation to this, it is an extremely potent agonist of the serotonin 5-HT_2A receptor in vitro, showing the highest potency of any other tryptamine assessed in one study. Its potency in activating the serotonin 5-HT_2A receptor was 38-fold higher than that of dimethyltryptamine (DMT) and 361-fold higher than that of psilocin in the same study. It is also a highly potent agonist of the serotonin 5-HT_2B receptor, with an EC₅₀ of 4 nM.

Whereas tryptamine, serotonin (5-hydroxytryptamine), and αMT show potent activity as monoamine releasing agents, including of serotonin, norepinephrine, and/or dopamine, the monoamine-releasing activity of 5-methoxylated tryptamine derivatives, like 5-methoxytryptamine, 5-MeO-NMT, and 5-MeO-DMT among others, is dramatically reduced or abolished. Accordingly, whereas the EC₅₀ values of αMT for induction of monoamine release are 22 to 68 nM for serotonin, 79 to 112 nM for norepinephrine, and 79 to 180 nM for dopamine, the EC₅₀ values in the case of 5-MeO-AMT are 460 nM for serotonin, 8,900 nM for norepinephrine, and 1,500 nM for dopamine. Similarly, it is of very low potency as a monoamine reuptake inhibitor (IC₅₀Tooltip half-maximal inhibitory concentration values >1,000 nM). Considering the very high potency of 5-MeO-AMT in activating the serotonin 5-HT_2A receptor, its weak activities as a monoamine releasing agent and reuptake inhibitor are of questionable significance.