It is biologically possible for smoking marijuana to increase cancer risk since marijuana smoke contains many of the same carcinogens as tobacco smoke. Also, people who smoke marijuana tend to inhale more smoke per puff and hold it in their lungs for longer than people who smoke tobacco cigarettes.
We need more evidence to know for sure about the possible cancer risk. Some studies suggest that using marijuana over a long period of time may increase the risk of cancer, particularly cancers of the lung, head and neck. However, the quality of this research is not as strong as the evidence on tobacco and cancer. Other studies do not show an increased risk of cancer after long-term marijuana smoking.
The fact that many people who smoke marijuana also smoke tobacco or mix the marijuana with tobacco makes it harder to know whether marijuana causes cancer. Also, because there is no standard amount of THC (the main active ingredient in marijuana) in every marijuana cigarette, it is hard to compare results across different studies.