It's very much the same with most illicit substance unfortunately. Because they are illegal the black market develops where substances that are used for legitimate purposes (i.e. opium/morphine) end up being cut with all kinds of materials so that dealers can get a better spread across their stock. It's a chain reaction really. Less quality in the product the more that people feel they have to use to get the 'best high' or use it in different way (ie shoot up, double drop pills etc).

There is some interesting research in Australia that looks at people who have bene arrested for all different kinds of offences (not specifically drug related). They ask for a urine sample and then get them to complete a survey about what drugs they believe they were taking that night. Compared to the urine results, the majority of people who thought that they had taken MDMA - were actually on methamphetamine.

David Nutt in the UK has done some interesting research/papers comparing the 'actual' harm of drugs vs the listing/scheduling of 'dangerous drugs' and has found that the way in which we categorise dangerous drugs has nothing to do with the actual harm that they cause (his research found that alcohol is by far the worst/harmful drug).