An antidepressant drug is a good example of this case. This drug is used primarily to treat clinical depression, but it has other uses as well. The drug may also be classified according to its mechanism of action. Prozac and Paxil are selective serotonin reuptake inhibitors that are classified primarily as antidepressant drugs that act on the neurotransmitter serotonin, while tricyclic antidepressants, such as Elavil, act on the three different neurotransmitters – serotonin, dopamine, and norepinephrine. Also, certain selective serotonin reuptake inhibitors are used to treat anxiety disorders and personality disorders, and may be classified as drugs that are designed for the treatment of those conditions.

Several organizations classify drugs based on the chemical composition or mechanism of action. The National Drug File – Reference Terminology classified drug according to four categories:

• Chemical structure of the drug
• Cellular mechanism of action of the drug, including its sub-cellular mechanism of action
• Therapeutic intent of the drug
• Organ or system level of the physiological effects of the drug

Drugs may also be categorized based on their shared effects, resulting in 7 to 10, or even more, major categories of drugs. Several drug manuals used this particular system of classification without regard to the potential abuse or addiction to the drug.

The International Drug Evaluation and Classification Program classified drugs into seven categories that are used in identifying and clarifying the types of drugs a person may have been taking. The categories of drugs are based on their major behavioral effects to allow the quick determination of the type of drugs a person may have been taking based on observations of his behavior. The  following are the seven major categories of drugs:

1. Central nervous system depressants – These are drugs that depress or slow down the functions of the body’s systems.
2. Stimulants – These drugs accelerate breathing, increase heart rate, and resulting in an increase in blood pressure and speeding up the overall processes of the body.
3. Hallucinogenic drugs – These are drugs that used mainly as hypnotic enhancers or sleep aides, affecting the CNS, have sedating effects, and alter the perception of reality.
4. Dissociative anesthetic drugs – They are used to inhibit the perception of pain.
5. Narcotic analgesic medication – These are medications that are used primarily for the treatment of pain. They are derived from opium or the poppy plant.
6. Inhalant drugs – These drugs are breathable substances that are traditionally not sold as medications or drugs marketed for that purpose.
7. Cannabis products – They include products containing the active ingredient THC. This group includes marijuana and hashish.