Pregnacy Category B (Animal reproduction studies have failed to demonstrate a risk to the fetus and there are no adequate and well-controlled studies in pregnant women OR Animal studies have shown an adverse effect, but adequate and well-controlled studies in pregnant women have failed to demonstrate a risk to the fetus in any trimester). Acyclovir was not teratogenic in mouse, rabbits, or rat studies. There are no adequate and well-controlled studies in pregnaant women.