Cannabinoids, the active chemicals in medical marijuana, are similar to chemicals the body makes that are involved in appetite, memory, movement, and pain. So far, solid studies suggest that these chemicals can:

• Control vomiting in people undergoing cancer chemotherapy. Drugs based on two lab-made forms of THC are FDA-approved for this purpose.
• Modestly reduce pain in people with conditions such as nerve damage, cancer, multiple sclerosis, and rheumatoid arthritis.
• Reduce muscle spasms in people with multiple sclerosis. 
• Improve sleep in the short term for people with obstructive sleep apnea, fibromyalgia, chronic pain, or multiple sclerosis.
• Increase appetite and decrease weight loss in people with HIV and AIDS.
• Reduce tics in people with Tourette’s syndrome.
• Reduce anxiety in people with social anxiety disorder.
• Improve symptoms of post-traumatic stress disorder.

References: https://www.webmd.com/a-to-z-guides/medical-marijuana-faq