A different drug altogether or a drug cut with harmful substances or contaminated with impurities.
Drug dealers are in business to make money.
They need to make lots of it to make their risks worthwhile. Keeping drugs pure costs money, so its not a very important consideration to dealers. Side effects include jittery feelings, irritability and palpitations. More serious side effects include sleep problems and decline in appetites for food and sex. The most serious problem is that these drugs are very addictive.