evidence: Marijuana is made from the dried leaves and buds of the Cannabis sativa plant. While the Food and Drug Administration (FDA) hasn't approved the use of the marijuana plant as medicine, many states have legalized marijuana use. [...] - Nausea and vomiting associated with cancer treatment. Research has shown that an active ingredient in marijuana, tetrahydrocannabinol (THC), effectively reduces nausea and vomiting in people undergoing chemotherapy. [...] However, marijuana use can cause cognitive impairment and should be used with caution if you have a mental health condition. In many places marijuana use is considered illegal for any purpose.