A defined daily dose, or DDD, is a measurement of prescription drug consumption created by the World Health Organization to help researchers when comparing the effects of different products. The name "daily dose" can be confusing; though the dose is measured in a specific quantity of grams, varying between drugs, this is not a recommended amount for a patient to take. Actual prescriptions will often vary, depending on the age and condition of the patient. However, the DDD measurement system allows researchers to calculate, using a simple algebraic equation, the number of defined daily doses in a given prescription.
Multiply the number of items issued by the amount of drug contained in each item. For example, a patient is given a prescription of 54 painkiller tablets, each containing 10 mg of ibuprofen. The prescription as a whole contains 540 mg of ibuprofen.
Search for the drug's DDD measurement on the WHO Collaborating Centre for Drug Statistics Methodology website (see Resources), by name or ATC code. Ibuprofen's DDD measurement, for example, is 30 mg.
Divide the total amount of the drug by the DDD measurement. In the ibuprofen example, this would give the equation (54 times 10 mg divided by 30 mg, yielding a final value of 18. This number is the drug usage, or number of defined daily doses, in the prescription: we have here a prescription containing 18 DDDs of ibuprofen.