There are degrees of drug dependence ranging from mild dependency to compulsive drug use (often referred to as addiction). It is impossible to say how long or how often a person must use a drug before they become dependent to it.

Dependence can be psychological or physical, or both.

People who are psychologically dependent on a drug feel compelled in certain situations, to use a drug in order to function effectively, or to achieve emotional satisfaction.

Physical dependence is when a person's body adapts to a drug and becomes use to functioning with the drug present.

If a physically and/or psychologically dependent person suddenly stops taking the drug, they may experience withdrawal symptoms as they readjust to functioning without the drug. Withdrawal symptoms are different for different types of drugs and for each person. There are numerous types of withdrawal symptoms that may be experienced. A person may experience symptoms such as depression, irritability, cramps, nausea, sweating and sleeping problems.

People who are physically dependent on a drug usually develop a tolerance to the drug. This means that they need to take more and more of the drug to get the same effect.