RT Journal
A1 Pasternak B, Hviid A
T1 RIsk of birth defects after use of acyclovir, valacyclovir, and famciclovir in the first trimester—reply
JF JAMA
JO JAMA
YR 2010
FD November 24
VO 304
IS 20
SP 2242
OP 2243
DO 10.1001/jama.2010.1702
UL http://dx.doi.org/10.1001/jama.2010.1702
AB 
In view of the difficulties in performing studies of drug safety in pregnancy because of the relative rarity of both exposure and outcome, the use of nationwide registry data allows the identification of a sufficient number of exposed pregnancies to reach adequate power, at least for birth defects in aggregate. Furthermore, antivirals are prescribed for acute infections, and women who filled prescriptions for these drugs are therefore more likely to have used the drugs compared with (as an example) those who filled prescriptions for drugs prescribed for prophylactic indications. Moreover, people who fill prescriptions for drugs seem very likely to have used them since they have traveled to a pharmacy and purchased the drugs.