2C-T-21.5, also known as 4-(2,2-difluoroethylthio)-2,5-dimethoxyphenethylamine, is a lesser-known psychedelic drug related to compounds such as 2C-T-21 and 2C-T-28. It was originally named by Alexander Shulgin and discussed in his book PiHKAL, but was not synthesised at that time. 2C-T-21.5 was ultimately synthesised and tested by Daniel Trachsel some years later. It has a binding affinity of 146 nM at 5-HT<sub>2A</sub> and 55 nM at 5-HT<sub>2C</sub>. It produces typical psychedelic effects, being slightly less potent but somewhat longer acting than 2C-T-2 or 2C-T-21, with an active dose of 12âÂÂ30 mg, and a duration of action of 8âÂÂ14 hours. Unlike 2C-T-21 it will not form the highly toxic fluoroacetate as a metabolite, instead producing the less toxic difluoroacetic acid. It is a controlled substance in Canada under phenethylamine blanket-ban language.