DRUGS, PREGNANCY, AND LACTATION.

Antihistamines comprise a large, fairly diverse group of compounds that are so prevalent that many women are probably exposed to these drugs at some point during pregnancy.

In general, first-generation antihistamines are usually safe during pregnancy when used occasionally, but should not be used for frequent, chronic allergy symptoms. There is little human pregnancy information on second-generation antihistamines.

During the first trimester, it is best to avoid antihistamines. But if an antihistamine is indicated, the best option is the oral first-generation drug chlorpheniramine, which is marketed over the counter under several names, including Chlor-Trimeton. If an injectable antihistamine is indicated, diphenhydramine (Benadryl)would be the first choice.

Most published data on antihistamine use during pregnancy concern these two drugs and indicate that they are relatively safe when used occasionally. There have been no signals or clusters of birth defects found in multiple studies of these agents.

The three second-generation antihistamines--cetirizine (Zyrtec), loratadine (Claritin), and fexofenadine (Allegra)--have not been shown to be teratogenic in animals. Nevertheless, they should be avoided if possible during the first trimester because of the sparse amount of data in humans on use of these drugs during pregnancy These data include a 1997 study of pregnant women that found no significant difference in the rates of major or minor malformations, gestational age at delivery, birth weight, or neonatal distress among 39 women who took cetirizine (most included first-trimester use), 81 women who took hydroxyzine (Vistaril or Atarax), and 110 controls who did not take either agent.

In a 1998 study of 18 women who took loratadine during the first trimester, 16 had normal infants with no major malformations; the other two had elective abortions. But this study is small and did not look for minor malformations. Loratadine is considered by many authors to be an acceptable alternative to a first-generation antihista mine, except during the first trimester.

To date, there have been no reports of human pregnancies exposed to Allegra, but problems with implantation have been reported in rats administered three times the human dose.