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Prescription Dose Changes - Are initially recommended doses too high?

August 13, 2002

Over 20% of prescription drugs undergo substantial reductions in their recommended dosages, relative to the doses initially recommended, according to two papers published in the August issue of Pharmacoepidemiology and Drug Safety, available online on 14 August 2002. These changes reveal a discrepancy between the dosage information gathered from pre-marketing studies and that needed for safe and effective use in clinical practice. Avoiding needless overdosing could reduce both side-effects and costs of prescription drugs.

In modern drug development, great pressure is placed on defining a drug dosage early in the process, when information about the drug and its actions is sparse. Ideally this dosage should remain the same throughout the lifetime of the drug, but these studies published this week indicate that the dosages of cardiovascular and most other classes of drugs are initially overestimated by a factor of two or more. These overestimated doses are discovered at various times after the drug is introduced into medical practice.

Researchers from the Georgetown University Center for Drug Development Science, Washington DC, USA, studied the label changes made for all the new drugs approved for use in the USA by the FDA (Food and Drug Administration) during the period 1980 -1999. The team led by James Cross found that dosage changes occurred in 21% of the drugs. Of these 79% were safety motivated changes. "This pattern may represent a systematic flaw in pre-marketing dosage evaluation," said Cross et al.

Evaluating the changes in Defined Daily Dose, compiled in many countries by the World Health Organisation, Dutch researchers from Utrecht University and Maastricht University, observed a similar proportion of dosage changes in drugs introduced in other countries. "The frequency of post-marketing reductions in dose increased during the 1990's, compared to the 1980's, so the problem appears to be getting worse, not better. It's not clear, however, whether we're getting better at finding doses that have been set too high, or if we making more mistakes in defining doses during drug development," said Eibert R. Heerdink, of Utrecht University.

The team let by Eibert R. Heerdink found that one class of drugs is distinctly different: antibiotics, which tend to need increasing doses as time passes after market introduction. This change presumably reflects gradually increasing resistance of micro-organisms to antibiotic action, as usage of the antibiotic increases.

Both papers offer several possible reasons for these changes. These include higher drug prices motivating tests of lower doses, changes in regulatory practice, and misjudgements made in the early phases of new drug development. All the researchers agree that more research needs to be done on the process of dose-selection, so that patients get the optimal dose right from the start.

In his editorial accompanying the issue Professor H. A. J. Struijker-Boudier observes, "chronic therapy with a good drug given at an inadequate regimen is ultimately poor drug therapy."

John Wiley & Sons

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