A new era of antiviral therapy was ushered in by acyclovir, because it is very selective and non-toxic. Acyclovir belongs to a class of drugs call nucleoside analogues. A nucleoside is a kissing cousin of the building blocks of DNA and RNA, namely nucleotides. When acyclovir is introduced into an infected cell, it prohibits the virus particles from reproducing.

Acyclovir is active against both the HSV-1 and HSV-2 strains for herpes simplex, as well as shingles, Epstein-Barr, and related viruses. Patients are seldom resistant to its effects, though resistance is more likely for victims of HIV or patients who received transplants and were treated with drugs to reduce organ rejection.

Side effects can include nausea, vomiting, diarrhea and/or headache. The list of side effects grows when the dosage is very high, and includes hallucinations, agitation, vertigo and dizziness. Acyclovir can be incorporated into a patient’s own DNA, and is therefore not generally recommended for use by pregnant women. A related drug, Valtrex, is marketed on television in the United States. It exhibits higher bio-availability than acyclovir.