People who use substances can develop a tolerance and so find that they need to use increased amounts to achieve the same effect. Many people who use substances have experience of this – including people who drink coffee, smoke tobacco or drink alcohol.

After regular repeated exposure to some substances, people report that they are not able to get the same effect that they had in the previously when they first used a substance or first used a substance after a long break from use.

Some people claim that the pursuit of this early effect has led them into problem use. This is sometimes called ‘chasing it’.

There are physical and psychological aspects to tolerance (see dependency) and it is not fully understood or explained by physiological effects and processes.

A period of abstinence or reduction in use can reverse tolerance. This makes stopping using and restarting a period of high risk of overdose. Changes in the function of vital organs through illness or ageing can also reduce tolerance. Many overdoses including fatal overdoses are related to people having lowered tolerance. This seems to be more of a factor than variations in the strength of drugs supplied though illegal unregulated supply routes.

Some practices in services, for example, pushing people out of treatment who ‘top up’ with street drugs seem irrational if they are viewed from a perspective of changed tolerance. Difficulty ensuring people transition immediately from prison or hospital treatment services to community-based treatment can leave people exposed to risk of overdose and death due to changed tolerance.

The reason that people are on different doses of OST medication is related to tolerance which is not, as many people seem to believe, a proxy measure of a person’s ‘addictedness’ or the extent of their drug problem or a measure of how close they are to abstinence or to recovery or ‘how well they are doing’(see optimal dosing ; see parked on methadone).