# Defined daily dose

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The defined daily dose (DDD) is a statistical measure of drug consumption, defined by the World Health Organization (WHO). It is used to standardize the comparison of drug usage between different drugs or between different health care environments. The DDD is not to be confused with the therapeutic dose or with the dose actually prescribed by a physician for an individual patient.[1]

The WHO's definition is: "The DDD is the assumed average maintenance dose per day for a drug used for its main indication in adults."[1]

## Use

A common problem when comparing drugs is that different medications can be of different potency. For example, 20 mg of the beta blocker propranolol are much less effective than 20 mg of the beta blocker bisoprolol.[2] To reflect this, the WHO has decided on a DDD for propranolol of 160 mg[3] and for bisoprolol of 10 mg.[4] Individual patients can still be prescribed higher or lower doses, for instance in children, patients with liver or kidney impairment, patients with a combination therapy, or due to differences in drug metabolism between individuals or ethnicities (genetic polymorphism).[1]

The DDD system is most frequently used in academic articles and reports, and as a tool for comparison and control over nationwide total drug consumption. For example, the overall drug consumption of beta blockers can be measured in DDDs and compared between different countries, sexes or other populations.

## Calculation

If the DDD for a certain drug is given, the number of DDDs used by an individual patient or (more commonly) by a collective of patients is as follows.

$Drug\ usage(DDDs) = \frac{Items\ issued \times Amount\ of\ drug\ per\ item}{DDD}$

For example, the analgesic (pain reliever) paracetamol has a DDD of 3 g, which means that an average patient who takes paracetamol for its main indication, which is pain relief, uses 3 grams per day. This is equivalent to six standard tablets of 500 mg each. If a patient consumes 24 such tablets (12 g of paracetamol in total) over a certain span of time, this equals a consumption of four DDDs.

$Drug\ usage(DDDs) = \frac{24 \times 500mg}{3g} = 4$