Drug Tolerance and Dependence: Understanding Addiction

Defining tolerance

Tolerance develops when the body adapts to repeated exposure to a drug, so that the same dose produces a smaller effect. The user must take a progressively larger dose to achieve the original effect. This is a physiological adaptation, not simply "getting used to" the drug in a casual sense.

Tolerance can develop to both legal drugs (such as alcohol and caffeine) and illegal drugs (such as heroin). The danger is that higher doses increase the risk of overdose and toxic side effects.

Physical and psychological dependence

Physical dependence occurs when the body has adjusted its normal chemistry to account for the presence of the drug. If the drug is suddenly removed, the body cannot function normally and the person experiences withdrawal symptoms. These vary by drug but can include tremors, nausea, sweating, pain and anxiety.

Psychological dependence means the person feels a strong emotional need for the drug, believing they cannot cope or feel normal without it. Both forms of dependence can exist together, making addiction extremely difficult to overcome.

The cycle of addiction

Addiction typically follows a pattern: initial use produces a pleasurable effect, tolerance develops so the dose increases, dependence means the person cannot stop without suffering withdrawal, and continued use despite harmful consequences. Breaking this cycle requires addressing both the physical and psychological aspects.

For IGCSE, the key exam skill is explaining the link between tolerance, dependence and withdrawal as a biological sequence rather than listing them as separate facts.

Examples across different drugs