Known chemically as O-desmethyltramadol. M1 is an opioid analgesic originally known as the main active metabolite of the weak opioid tramadol. M1 has been marketed as a research chemical and is considerably more potent than tramadol as a mu agonist. It shows additional activity as a noradrenaline reuptake inhibitor, a 5HT2A antagonist and a muscarinic acetylcholine antagonist.
When sold as a research chemical, M1 typically appears in powder form, or as a mixture with certain herbal blends (kratom, etc).