To the Editor: Dr Pasternak and colleagues1 concluded that there was no association between exposure to acyclovir or valacyclovir in the first trimester of pregnancy and the risk of major birth defects in a large nationwide cohort, inclusive of all live-born infants in Denmark from January 1, 1996, to September 30, 2008. One limitation of this study is that data on exposure were inferred from records of prescriptions filled, which may call that conclusion into question.
Hou B, Zhang Y. Risk of Birth Defects After Use of Acyclovir, Valacyclovir, and Famciclovir in the First Trimester. JAMA. 2010;304(20):2242-2243. doi:10.1001/jama.2010.1701