MDMA is structurally related to amphetamines. One important difference between MDMA and amphetamines is the presence of the methylenedioxy group (-O-CH2-O-) attached to the aromatic ring. This attachment makes it also resemble the structure of the hallucinogen mescaline. Like amphetamines, MDMA is a synthetic substance that does not exist in nature.

Similarly to amphetamines and cocaine, MDMA can exist as a free base or as salts of various acids. Unlike these drugs, however, MDMA tends not to be inhaled in its free base form. This is because the methylenedioxy group raises the boiling point of the free base so high that it becomes too difficult to use in such a manner (Shulgin, 1986).

The salts are not volatile, but are quite soluble in water and thus can be administered intravenously, orally or intranasally. 'Ecstasy' tablets sold on the street do not always contain MDMA, but may contain methylenedioxyethylamphetamine (MDEA), methylenedioxyamphetamine (MDA), paramethoxyamphetamine (PMA), ephedrine, ketamine or a range of other compounds (Becker, Neis, Rohrich & Zorntlein, in press; Byard, Gilbert, James & Lokan, 1998; Holden & Jackson, 1996).