Because street Ketamine usually originates from legitimate pharmaceutical supplies that have been illegally diverted to the black-market, the question of purity and actual chemical content is not generally as much of a concern as it is with other street drugs such as Ecstasy. However non-medical users of ketamine often comment that they notice varying effects from different batches of street ketamine. Although set and setting can have a dramatic influence on one's subjective experience with any psychoactive, there may be other reasons why some ketamine users feel one bag of ketamine may differ from the next.

According to an article written by Karl Jansen in the Journal of Psychoactive Drugs, Ketalar ® [produced by Parke-Davis] contains benzethonium chloride as a preservative, which may have its own psychoactive effect. In addition, some brands differ in which stereoisomers of ketamine they contain. Most brands contain equal mixtures of the S(+) and R(-) isomers (racemic ketamine), but at least one brand has been found to contain only the S(+) isomer. According to Jansen, S(+) ketamine is more likely to suppress breathing, induces a faster loss of consciousness, and has a quicker recovery time (Jansen, 2000). Jansen also states that S(+) ketamine is a more potent psychedelic drug than R(+) ketamine, but he does not elaborate as to how.