There is significant evidence that marijuana is carcinogenic, however exactly how much so is a matter of debate. There have been studies showing that marijuana smokers are at increased risk of lung cancer, cigarette smokers are well established as having a much higher risk of lung cancer, but smokers of marijuana and cigarettes have a lower risk of lung cancer than smokers of only cigarettes. Marijuana may have some cancer inhibitory effect as well as being carcinogenic, I have not seen any conclusive evidence as to why users of marijuana and cigarettes are at lower risk of lung cancer than users of only cigarettes. Additionally most marijuana smokers also are cigarette smokers. There was a study saying that marijuana use in childhood leads to lowered IQ through adulthood, but there was another researcher who pointed out flaws in this study so it is not really conclusive. There is strong evidence that marijuana can trigger latent psychological disorders in those who are susceptible, much like pretty much all psychedelics. Marijuana can lead to panic attacks as well, and panic attacks are imo the most negative effect most users are likely to experience. Overall marijuana is quite a safe drug, if you eat it or vaporize it or even use a water pipe of some sort you can eliminate or greatly reduce the increased risk of cancer. This leaves you with the risk of triggering a latent psychological disorder, but if you do not have a family history of mental disorders the risk of this happening is quite small. You could possibly become mentally dependent on marijuana, I have seen this happen to a few people but they are few and far between. There is some risk that you could become demotivated as well, but this is also not particularly common in my experience, I know a lot of highly accomplished marijuana smokers. So in summary: A. You could increase your risk of cancer, but this can be eliminated or greatly reduced depending on the way you consume marijuana. Also, it is apparently safer to smoke marijuana and cigarettes than to smoke only cigarettes. B. You could trigger a latent psychological disorder, schizophrenia is of particular concern. The chances of this happening if you don't have a family history of mental illness are quite small. C. You could come to depend on marijuana to not be bored, or have mental cravings for marijuana and feel uncomfortable when you are not high, but this is fairly rare and I have only seen it effect the heaviest of users. D. You could lose motivation, but this is pretty rare as well and there are lots of accomplished doctors, scientists and others who have mentally and physically demanding jobs who use marijuana. This is also much more likely to effect heavy users than light to moderate users.  E. You could suffer from quite intense panic attacks while under the influence, this is not particularly rare and it seems like it effects quite a lot of marijuana users at one time or another. Some people are more sensitive than others. The panic goes away when the high does, and you can probably stay in control of yourself, and if you keep some fast acting benzo on hand you will be even better off. F. Driving skills are negatively effected while under the influence of marijuana, but not to the same extent as when under the influence of alcohol. I think that is a pretty comprehensive list of the negative health effects of marijuana, and as you can see many of them are preventable / reducible with responsible use and thoughtful planning.