The defined daily dose (DDD) is a statistical measure of drug consumption, defined by the World Health Organization (WHO) Collaborating Centre for Drug Statistics Methodology.
[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.
[2] Before a DDD is assigned by the WHO Collaborating Centre for Drug Statistics Methodology, it must have an ATC Code and be approved for sale in at least one country.
The DDD of combination tablets (containing more than one drug) is more complex, most taking into account a "unit dose", though combination tablets used for high blood pressure take the number of doses per day into account.
[1] The formula for determining the dose is:[1] The DDD of a drug is reviewed after three years.
[3] The DDD is generally the same for all formulations of a drug, even if some (e.g., flavoured syrup) are designed with children in mind.
Some types of drug are not assigned a DDD, for example: medicines applied to the skin, anaesthetics and vaccines.
[1] Different people may in practice be prescribed higher or lower doses than the DDD, for instance in children, people 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 is often use for long term research and analysis of drug utilisation trends over time, so changes to the DDD are avoided if possible,[3] whereas changes in the actual daily dose prescribed for a population may often occur.
The indicator DDDs per inhabitant per year can be used for drugs normally prescribed for short treatment duration (e.g., antibiotics) to indicate the average number of days in a year a person may take that treatment.
They recommend that "DDDs are not suitable for comparing drugs for specific, detailed pricing, reimbursement and cost-containment decisions".
The average cost per DDD of rosuvastatin was 21% more expensive than atorvastatin ($1.14 compared to $0.94), which would suggest the shift at the time from prescribing atorvastatin to prescribing rosuvastatin would result in increased costs to the healthcare budget.
If one compared cost per unit (daily tablet) then rosuvastatin was instead 24% cheaper than atorvastatin ($1.44 vs $1.90), and if one compares cost per RDD (recorded daily dose) then rosuvastatin was 26% cheaper than atorvastatin ($1.43 vs $1.93).
[4] The Canadian Patented Medicine Prices Review Board analysed the use of DDD for drug utilisation and cost analysis and offered recommendations.
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.