Definition:
Drugs can be considered harmful when their level of use causes physical, mental, social or economic problems. But even when used under medical guidance, some drugs can have undesirable side effects. Many drugs may also have effects beyond those for which the user is looking. When such drugs are used non-medically, these side effects become potentially dangerous.

Safety Hazards:
All psychoactive drugs can reduce physical co-ordination, distort the senses, or impair judgment. These effects can lead to serious safety risks, especially if the user drives a motor vehicle or operates machinery.  Drivers intoxicated by alcohol or some other drug can cause many road injuries and fatalities.  Often people who have taken alcohol or drugs are unaware of their impairment, which makes the risk all that much greater.

Physical Health Hazards:
All psychoactive drugs have effects other than those for which they are used, and some of these can be very damaging to physical health. Smoking marijuana or tobacco, for example can cause lung damage. Sniffing cocaine can damage the inside of the nose. Users who inject drugs by hypodermic needles can get infections such as serum hepatitis or AIDS. Mental Health Hazards: Some drugs can cause short-term confusion, anxiety or even severe mental disturbances caused by "Bad trips". In the longer term, drug abuse can result in personality disturbances, learning problems, and loss of memory. Mental health risks are especially high for young drug users. A young person who turns to drugs as a way of avoiding normal anxiety and depression may be establishing a pattern of behavior that can be hard to break in the future.

Physical Dependence:
This occurs when a drug users body becomes so accustomed to a particular drug that it can only function normally if the drug is present. Without the drug the user may experience a variety of symptoms, some of which can be fatal, that are collectively referred to as "withdrawal".  Not all drugs produce physical dependence, but they may still be abused because of their perceived effects, and as a result of psychological dependence. Physical dependence is one of the factors contributing to the continued use of drugs.

Psychological Dependence:
This dependence exists when a drug is so central to a persons thoughts, emotions, and activities that it is extremely difficult to stop using it, or even stop thinking about it. Like physical dependence, psychological dependence is a cause of continued drug use.

Tolerance:
Tolerance means that, over time and with regular use, a user needs more and more of a drug to get the same effect. Tolerance increases the physical health hazards of any drug simply because it can result in increased drug use over time. Tolerance also increases the risk of a dangerous or fatal overdose, for two reasons:   

1. With some drugs, the body does not necessarily develop tolerance to all the effects of the drug to the same extent. Long-term barbituate users for instance become tolerant to the mood altering effect of the drug, but less tolerant to its depressant effect on respiration. When this happens, the dose required to achieve this effect may be dangerously close to the lethal dose.
2. If a drug user has not taken the drug in a long time, the expected tolerance may actually have decreased. So, after a long period of abstinence, the size of the dose the user had previously become accustomed to may actually be enough to cause an overdose.

Street Drug Hazards:
Illegal street drugs have a set of risks all their own.
Users of street drugs can never know exactly what they are taking.
Dealers may not know (or reveal) exactly what they are selling.
Some drugs are laced with other drugs or chemicals, which can be harmful. Often one drug is sold in the place of another. Research and testing of street drugs has revealed that two thirds of the drugs that were bought turned out not to be what the purchaser was buying, therefore you don't know what you are buying over half the time. Many bad drug reactions, including fatal overdoses, are caused by the user's ignorance of exactly what drug and how much of it they are taking.