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Prevalence

The term prevalence refers to the proportion of a population who have used a drug over a particular time period. In general population surveys, prevalence is measured by asking respondents in a representative sample drawn from the population to recall their use of drugs.

In the NACD drug prevalence surveys the following terms are used:

The three most widely used recall periods are: lifetime (ever used a drug), last year (used a drug in the last twelve months), and last month (used a drug in the last 30 days). Provided that a sample is representative of the total population, prevalence information obtained from a sample can be used to infer prevalence in the population.

• Lifetime prevalence refers to the proportion of the sample that reported ever having used the named drug at the time they were surveyed. A person who records lifetime prevalence may or may not be currently using the drug. Lifetime prevalence should not be interpreted as meaning that people have necessarily used a drug over a long period of time or that they will use the drug in future.

• Last year prevalence refers to the proportion of the sample that reported using a named drug in the year prior to the survey. Last year prevalence is often referred to as recent use.

• Last month prevalence refers to the proportion of the sample that reported using a named drug in the 30 day period prior to the survey. Last month prevalence is often referred to as current use. A proportion of those reporting current use may be occasional (or first-time) users who happen to have used in the period leading up to the survey. It should therefore be appreciated that current use is not synonymous with regular use.

When examining the data and comparing results over time, last year use is the best reflection of changes as it refers to recent use. Last month use is equally valuable as it refers to current use.

Related terms:incidence

NACD prevalence bulletins

National Institute for Health and Care Excellence