With marijuana being recently legalized in California, questions about its positive and negative effects on health overall and mental health in particular have intensified. There is an appropriate amount of debate around it and a growing body of investigative research attempting to arrive at conclusive findings. But in short, we really don't know yet.
My overall take on the state of the literature is that, like with any other foreign substance that you are introducing into your body, avoid or limit it if you can. However, compared to other drugs that are commonly abused - and particularly alcohol - marijuana has a lower profile of long-term damage and side effects.
The article below does a good job of listing areas of interest in the research of clinical uses of marijuana and where we are in our understanding of them. For example, while we all know that marijuana's THC can have a 'feel good' effect on the brain, it also elevates heart rate and impacts coordination and balance. Cannabidiol (CBD) has been demonstrated to help in pain management, but marijuana can also affect memory, mood, and potentially activate schizophrenic symptoms in those prone to the condition.
If you are considering or actively using marijuana for mental health concerns, I suggest discussing it with your doctor to investigate any potential physical health risks and with a therapist to learn additional or alternative ways to manage your pain or mood.
All said, natural, healthy, and drug-free solutions are always best!