It is a system used by the U.S. Food and Drug Administration (FDA) to categorize the risk of fetal harm associated with the use of medications during pregnancy.
The categories are as follows:
Category A: Adequate and well-controlled studies have failed to demonstrate a risk to the fetus in the first trimester of pregnancy (and there is no evidence of risk in later trimesters).
Category B: Animal reproduction studies have failed to demonstrate a risk to the fetus, but there are no adequate and well-controlled studies in pregnant women.
Category C: Animal reproduction studies have shown adverse effects on the fetus, and there are no adequate and well-controlled studies in humans, but potential benefits may warrant use of the drug in pregnant women despite potential risks.
Category D: There is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience, but potential benefits may warrant use of the drug in pregnant women despite potential risks.
Category X: Studies in animals or humans have demonstrated fetal abnormalities, and/or there is positive evidence of human fetal risk based on adverse reaction data from investigational or marketing experience, and the risks involved in use of the drug in pregnant women clearly outweigh potential benefits.
It's worth noting that the FDA has moved away from this system in recent years, and labeling now includes more detailed information about the risks associated with drug use during pregnancy rather than simply assigning a category.
Further Reading:
Bledsoe, B. E. & Clayden, D. (2018) Prehospital Emergency Pharmacology (8th Ed). Pearson.
Guy, J. S. (2019) Pharmacology for the Prehospital Professional (2nd Ed) Jones & Bartlett Learning.
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