Drugs that act on the nervous system, such as the brain, and impact a person's mental state may be loosely and informally classified into categories. The following categories are not fully agreed upon globally:
- Hard drugs are drugs that lead to physical and psychological addiction and potentially death. Making, selling, or using drugs other than for approved medical purposes is illegal in most countries. Hard drugs include opioids like heroin, hydrocodone, oxycodone (Oxycontin) and morphine. Benzodiazepines are another class of hard drugs and include diazepam, alprazolam, clonazepam, and lorazepam. Hard stimulant drugs include methamphetamine (meth), cocaine, and nicotine. Drugs such as alcohol and nicotine are legal to sell in many countries and are often taxed.
- Soft drugs are not thought to cause physical or psychological addiction or dependence to the extent of hard drugs, however they are still considered unsafe. Examples of soft drugs are hallucinogens like cannabis, mescaline, psilocybin, LSD, ayahuasca, iboga, and DMT. While they do not cause physical addiction, some of them may still lead to psychological dependence. There is some evidence suggesting some hallucinogens have therapeutic uses.
- Some drugs do not fit perfectly as either a soft or hard drug and have characteristics of both. Examples for such drugs are MDMA (known as Ecstasy/Molly), ketamine, PCP (phencyclidine), DXM (dextromethorphan), synthetic cannabis (known as Spice or K2), and caffeine.