Per Thomas W. Hale, PhD (Medications and Mothers’ Milk, 2002, p.15):
Pregnancy Risk Categories have been assigned to almost all medications by their manufacturers and are based on the level of risk the drug poses to the fetus during gestation. They are not useful in assigning risk via breastfeeding[emphasis added]… Unfortunately they do not indicate the importance of when during gestation the medication is used, since some drugs are more dangerous during certain trimesters of pregnancy.
|A||CONTROLLED STUDIES SHOW NO RISK:
Adequate, well-controlled studies in pregnant women have failed to demonstrate a risk to the fetus in any trimester of pregnancy.
|B||NO EVIDENCE OF RISK IN HUMANS:
Adequate, well-controlled studies in pregnant women have not shown increased risk of fetal abnormalities despite adverse findings in animals, or, in the absence of adequate human studies, animal studies show no fetal risk. The chance of fetal harm is remote, but remains a possibility.
|C||RISK CANNOT BE RULED OUT:
Adequate,well-controlled human studies are lacking, and animal studies have shown a risk to the fetus or are lacking as well. There is a chance of fetal harm if the drug is administered during pregnancy; but the potential benefits may outweigh the potential risks.
|D||POSITIVE EVIDENCE OF RISK:
Studies in humans, or investigational or post-marketing data, have demonstrated fetal risk. Nevertheless, potential benefits from the use of the drug may outweigh the potential risk. For example, the drug may be acceptable if needed in a life-threatening situation or serious disease for which safer drugs cannot be used or are ineffective.
|X||CONTRAINDICATED IN PREGNANCY:
Studies in animals or humans, or investigational or post-marketing reports, have demonstrated positive evidence of fetal abnormalities or risks which clearly outweighs any possible benefit to the patient.
A few sources of information on medication use during pregnancy
Drugs in Pregnancy and Breastfeeding from perinatology.com
Motherisk (phone 416-813-6780) at The Hospital for Sick Children in Toronto, Ontario, Canada. Call or visit their website for evidence-based information about the safety or risk of drugs, chemicals and disease during pregnancy and lactation.
Drugs in Pregnancy and Lactation: A Reference Guide to Fetal and Neonatal Risk by Gerald G. Briggs, Roger K. Freeman and Sumner J. Yaffe