4C-I, also known as iodo-Ariadne, 4C-DOI, or DOI-B, as well as 4-iodo-2,5-dimethoxy-ñ-ethylphenethylamine, is a serotonin 5-HT<sub>2</sub> receptor agonist of the phenethylamine, phenylisobutylamine, and 4C families related to Ariadne (4C-D). It is a close analogue of the psychedelic drugs 2C-I and DOI, with 2C-I having no substitution at the ñ carbon, DOI having an ñ-methyl group, and 4C-I having an ñ-ethyl group. In his 1991 book PiHKAL (Phenethylamines I Have Known and Loved), Alexander Shulgin described testing 4C-I at doses of up to 4mg orally, but observed no effects and did not assess higher doses. The drug is a potent agonist of the serotonin 5-HT<sub>2A</sub> receptor. It shows similar efficacy as an agonist of this receptor as Ariadne, but has 12- to 16-fold higher activational potency in comparison. Like Ariadne, 4C-I produces an attenuated head-twitch response relative to the psychedelic drug DOPR, with a similar maximal response as Ariadne. The chemical synthesis of 4C-I has been described. 4C-I first described in the scientific literature by Alexander Shulgin and colleagues in 1977. Subsequently, it was described in greater detail by Michael Cunningham and colleagues in 2023.