Is there a such thing as medical marijuana? If so which doctor prescribes? What affect does long term marijuana do to the body?

Not in Texas. If you decide this is for you, don't expect any physician in Texas would risk his/her license to prescribe it for you. Move to Colorado or another permissive state and you can find out what their restrictions are.
Depends on where. A number of states have passed "medical marijuana" laws, but the problem is that marijuana is a plant where you do not know the purity, whether it contains other substances (like being sprayed with insecticides etc.). Some regulation to improve this is happening in those states, but the purity of the marijuana is always going to be variable. Marijuana can cause lung and other problems w heavy use.
Medical. Medical marijuana refers to the use of marijuana for medicinal reasons. 16 states (including the district of columbia) have medical marijuana laws. In those states, medical marijuana must be recommened by a physician for a particular condition that is covered under that state's law. Medical marijuana cards are issued by the public health department in those states. The state of texas does not have a medical marijuana registry - so it can not be legally prescribed to you if you live in la marque, texas.