This is not a well-phrased question. "Drugs" are not illegal. Some drugs, like caffeine, are completely legal; some drugs, like nicotine, alcohol, and pharmaceuticals are controlled -- you have to meet certain conditions in order to obtain them, like turning 18 or having a prescription; and some drugs, like marijuana, LSD, and crystal methamphetamine are banned.
Among the banned drugs, some, like crystal meth, create a danger to the person with no medical benefit. Some create a danger to others, like PCP. Some controlled drugs, like salvia in California or alcohol, have no medical benefit and are at least as dangerous as some banned drugs, but these types of drugs are heavily supported by money from the industries that produce them, and will never be banned.