Known brand names include Zovirax.
An antiviral for herpes viruses. Tablets treat shingles, chickenpox, and genital herpes, and the IV form treats severe herpes infections, including herpes of the brain and herpes in newborns. It can harm the kidneys, especially when given IV, so good hydration matters.
Inside cells infected with a herpes virus, a viral enzyme called thymidine kinase changes acyclovir into an active form, and the cell's own enzymes finish the job. The active form blocks the enzyme the virus uses to copy its DNA, and it also gets built into the new viral DNA and stops the strand from growing. Because the first step needs the viral enzyme, the drug acts mostly in infected cells. It works more strongly against herpes simplex than against the chickenpox and shingles virus, because the herpes simplex enzyme activates it more efficiently.
Only a small share of a tablet dose is absorbed, and larger doses are absorbed less efficiently, while the IV form reaches much higher blood levels. With healthy kidneys, half of it leaves the blood in about 2.5 to 3.3 hours, and the kidneys remove most of it unchanged in the urine. Because it dissolves poorly, it can form crystals in the kidney tubes when given too fast, in high doses, or to someone who is dehydrated, and this can lead to kidney failure. Herpes viruses can become resistant to it, mainly in people with weak immune systems such as those with advanced HIV.
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Acyclovir treats infections from herpes simplex virus and from varicella-zoster virus, the virus behind both chickenpox and shingles. The tablets treat shingles, chickenpox, and first and repeat outbreaks of genital herpes, a sexually transmitted infection. The IV form treats herpes simplex encephalitis (a dangerous herpes infection of the brain), herpes in newborns and infants, severe first outbreaks of genital herpes, and herpes or shingles in people with weak immune systems. It shortens outbreaks and helps the sores heal faster, but it does not cure herpes.
The tablets are swallowed with or without food, and they work best when started as soon as possible after symptoms appear. Their safety and effectiveness have not been shown in children younger than 2. The IV form drips slowly into a vein and must be given along with enough fluids to protect the kidneys. It has not been shown to stop genital herpes from spreading to sex partners, and the virus can pass even when there are no sores.
Nausea, vomiting, diarrhea, headache, and feeling generally unwell are the most common complaints with the tablets, while the IV form often irritates the vein it runs into. The label treats kidney damage and kidney failure, sometimes fatal, as a serious risk, especially with the IV form. It also warns about confusion, hallucinations, shaking, and seizures, which are more common in older adults and people with kidney problems, along with severe skin reactions and a sometimes deadly blood clotting disorder (TTP/HUS) in people with weak immune systems.