Why people shouldn't do drugs?

Well theres a number of reasons why people shouldn't do drugs.

In anything, theres always the risk of addiction (physical addiction in some things but anyone can get mentally addicted to anything.)
Amphetamines- From amphetamine abuse, users suffer from increased aggression, decreased appetite (which can result in massive vitamin deficiencies), insomnia (which can result in numerous bad things such as psychosis, imbalance of serotonin, ect.), and high risk of cardiac arrest from high blood pressure.
Opiates- Opiates come with an extremely high risk of addiction. Users can suffer from pulmonary edema (fluid accumulation in the lungs resulting in respiratory failure), respiratory despression, coma, or cardiac failure.
Coca-derivates (Cocaine and Crack)- Cocaine addiction can result in physiological damage, lethargy, psychosis, depression, or a potentially fatal overdose. Crack is much easier to get addicted to as it's the freebase form of cocaine. Chronic cocaine can degrade the nasal septum (cartillage separating the nostrils) and can lead to it's disappearance. Cocaine may also greatly increase this risk of developing rare autoimmune or connective tissue diseases such as Lupus, Goddpasture's Disease, vasculitis, glomerulonephritis, Stevens-Johnson syndrome, ect. Among those it can cause substantial damage to the kidneys.
Marijuana- Studies have shown that marijuana use can lead to despression, psychosis, schizophernia, bronchitis, and emphysema in people who are vulnerable to such illnesses by family or personal history. Although marijuana contains the same carcinogens as tobacco smoke, it's use cannot be linked to lung cancer. Cannabis use can result in increased appetite and highered blood pressure.
That covers a lot of things...