One negative effect marijuana has on our body is that it can increase the possibility of cancer. It can also affect your brain and the way you think. You could possibly become addicted. If you take marijuana when you’re pregnant, it could also affect your baby. Some people take marijuana to relieve pain during chemo, but if you’re using it for that reason, it’s not really curing cancer, it’s making it worse by putting a cancer causing substance in your body. It can also make you have paranoia and hallucinations.