One of the disadvantages of qualitative drug research is that you can never know just how reliable your participants are. Some subjects in a qualitative research study may not be credible. These subjects may claim to have experience with whatever drug the research is on, but simply lie to get research prize money. These kinds of participants will produce descriptions of drug experiences that have no validity. In other cases; participants may submit false or misleading background information. Participants who do so may skew the demographic section of the research results (e.g. create false figures for statements like "X percent of people from town Y experimented with this drug").
Qualitative research is less precise and more subjective compared to quantitative research. Qualitative research on drugs relies on user accounts of various aspects of a drug: effects, withdrawal symptoms and health consequences. Because these kinds of reports are so subjective, it can be hard to develop a method for cataloging the research data. One way of doing so is to "quantify" the qualitative findings, converting long descriptions of effects into, for example, 1 to 10 ratings. However, this method belies the whole idea of "qualitative" research to some extent. The other method is simply to read all the interview transcripts and reports, then summarize the data; findings presented this way add little to the raw material except a bit of organization and commentary.
The findings of qualitative drug research may be less useful than the findings of quantitative research. Quantitative findings (e.g. use rates, overdose rates, cities with the most users) can help police and medical professionals identify and find people who have drug problems and reform them before its too late. Qualitative findings (e.g. "the feeling of nausea during heroin withdrawal") have less use to professionals because they vary widely from user to user, and because even the most reliable accounts of how drugs feel to users don't tell professionals about how to treat users with drug problems.
Researcher bias can have a detrimental effect on the quality of qualitative research. With quantitative research, it can be difficult for the researcher to hide facts they do not like. Numbers are numbers and, while a biased researcher can skew statistics with disingenuous presentation, the raw findings are always there for outside observers to view. With qualitative research, there is no such safeguard. A researcher can ask leading questions to get research participants to say what he wants to hear so that, in the end, there is no way for a savvy observer to determine the findings the researcher is trying to conceal. As well, a qualitative researcher always has the ability to find research subjects that are affiliated with his cause (e.g. pro drug advocates, pharmaceutical lobbyists), thus, is more likely to say what he wants them to say.