What is medical marijuana?
Currently, the two main cannabinoids from the marijuana plant that are of medical interest are THC and CBD.
THC can increase appetite and reduce nausea. THC may also decrease pain, inflammation (swelling and redness), and muscle control problems.
Unlike THC, CBD is a cannabinoid that doesn't make people “high.” It may be useful in reducing pain and inflammation, controlling epileptic seizures, and possibly even treating mental illness and addictions.
Source: Marijuana as Medicine