What happens if you do drugs or drink alcohol while you have cancer?

Can't answer. Too broad a question. How much alcohol and which drugs? And, what kind of cancer? Social use of alcohol (1-2 drinks per day at most) has little effect on causing cancer, although when combined with smoking cigarettes, it does increase chances. If you are being treated for cancer, alcohol may interfere with the drugs used to treat it. Different drugs have different effects.
Metabolism abnormal. Drugs or alcohol are processed (metabolized) by the liver. This may change the livers ability to metabolize chemotherapy drugs.