This is a very vague question, different drugs cause different things, some cause nothing at all. Tobacco can cause lung cancer, emphysema, heart disease, high blood pressure, or gum infection....
99% of illegal drugs will show up on a full scale lab test as well as most prescription drugs. Illegal and prescription drugs are usually made from similar substances.
Drug tolerance can increase the likelihood of drug overdose for several reasons:
1) A person addicted to a drug to which they've built tolerance may overestimate the amount of the drug they need to...