CHAPTER 23: Purity, Potency, Drug Content and Prices

The quality of powdered drugs at street level has varied considerably over time; although the overall mean purity of heroin is typically between 40 and 50%, the mean purity of amphetamine is around 5–10% while the overall mean purity of cocaine has varied from 20 to over 60%. The quantity of active constituent present in a powder or tablet as well as the price of drugs is used for various casuistic or intelligence purposes, but drug content, purity and potency are not synonymous and the concepts are fraught with difficulties. For cannabis, purity cannot be used to express the THC content because if the sample contained 100% THC it would no longer be cannabis. A tablet might contain 150 mg of pure MDMA with a total weight of 300 mg. But to say it is 50% pure is almost meaningless since the overall weight reflects the tablet size. Even when applied to a powder, without careful handling the concept of purity can lead to confusion. For basic drugs (i.e., almost all substances of interest), most forensic science laboratories determine base drug purities. Thus, pure amphetamine sulfate has a base purity of only 73%, but a purity of 100% as amphetamine sulfate. The difference of 27% is accounted for by the sulfate part of the salt. Finally, more problems of interpretation arise with ‘average’ values. While this often refers to the mean, other measures of central tendency such as the weighted mean, the mode and median may be more appropriate.