There are difficulties in researching the effects of cannabis.

1. Many people who smoke cannabis also smoke tobacco. And users of cannabis often mix it with tobacco. This can make it difficult to know whether it is the tobacco, the cannabis, or both that has caused a cancer.
2. The amount of THC [4] in cannabis also varies. Some of the cannabis available today is much stronger than it was 20 years ago. These versions contain more THC.
3. Another difficulty researchers have is in recruiting people who smoke cannabis into studies. Because cannabis is an illegal drug in many countries, people may be reluctant to take part in research. And if they do agree to take part, they may not say how much cannabis they actually smoke.