2C-I is a psychedelic drug and phenethylamine that was developed and popularized by Alexander Shulgin. Its full chemical name is 2,5-dimethoxy-4-iodophenethylamine. It was described in Shulgin’s book PiHKAL. The drug is used both recreationally and as an entheogen but no medical or industrial uses have been reported yet. It is mostly commonly encountered in the form of its hydrochloride salt, a fluffy, sparkling white powder, and has also been pressed into tablet form. As it has only recently grown popular, slang terms for 2C-I vary with location, and it has been called "Tsi," "eyes," "ice," "butterfly," "twombly" (artist Cy Twombly, rhyming slang), "Deucey(I)" — a play on "Lucy," a common term for LSD, — and "hyperglitter," among others. 
2C-I is almost always taken orally, although it can also be insufflated, smoked, or administered rectally as well (though as with all phenethylamines of the 2C class, 2C-I is incredibly painful upon insufflation). There have also been a few reports of intramuscular and intravenous injections. An oral recreational dose of 2C-I is commonly between 10mg and 25mg, although doses as low as 2mg have been reported to be active. The onset of effects usually occurs within an hour, and the effects of the drug typically last somewhere in the range of 5 to 10 hours. The effects of the drug are often described as quite similar to those of its chemical relative 2C-B, combining psychedelic or hallucinogenic effects typical of drugs such as LSD with the empathogenic or entactogenic effects of drugs such as MDMA (ecstacy). Some users report that the effects are more mental and less sensory than those of 2C-B. Users of 2C-I do, however, tend to report a physical stimulant effect, often quite strong. Although unpleasant physical side effects such as muscle tension, nausea, and vomiting have been reported, their incidence in the use of 2C-I appears to be less common than in the use of some of the other closely related phenethylamines such as 2C-T-2 and 2C-T-7. User reports have said that 2C-I may produce flashbacks in the weeks following its use. These flashbacks can last anywhere between seconds and hours, and manifest as a return of the hallucinogenic effects of the drug. Some users report being able to trigger the flashbacks at will. Note however that these flashbacks do not occur in the majority, but have simply been observed in a selection of users. They are thought to be similar in nature to LSD flashbacks, and not particularly harmful or even remarkable (for a psychedelic drug). It may be interesting to note that experienced users of both LSD and 2C-I often state that the hallucinatory experience produced by both drugs is remarkably similar (this is interesting because different substances are known to produce tangibly different feelings during intoxication).