Tetrahydrocannabinol or THC is the primary active chemical in marijuana and is responsible for its psychotropic effects. In its pharmaceutical preparation, it goes by the name of "dronabinol". It is a fat in its natural state and it's believed that it discourages herbivores from eating the plant. It may also protect the plant from ultraviolet radiation.
Its primary medical use is the treatment of anorexia, nausea, and vomiting, usually during chemotherapy. It is also used to treat the symptoms of multiple sclerosis. It's generally safe at therapeutic doses, and overdoses are usually not fatal, although they can cause lethargy, decreased motor skills, slurred speech, and fainting.
The pharmaceutical preparation of THC is much slower acting than smoking marijuana, and dosage is hard to control as well. However, marijuana contains well over 100 substances that can affect human metabolism and its effect on a particular patient can be unpredictable as a result.