Can acyclovir resistance happen in real patients?
Yes. Herpes viruses can develop resistance to acyclovir, though it is uncommon in people with normal immune systems and becomes more likely in those with weakened immunity or prolonged antiviral use [1][2].
Why does resistance to acyclovir occur?
Acyclovir targets viral DNA replication. Resistance typically happens when the virus changes so the drug no longer works effectively. The best-known mechanism involves mutations in the viral thymidine kinase gene (UL23), which reduces activation of the drug inside infected cells, and/or changes in viral DNA polymerase (UL30) that make the replication machinery less affected by acyclovir [1][2].
Who is at higher risk of developing resistance?
Risk is higher when acyclovir is used for long periods or repeated suppressive therapy, especially in people with compromised immune systems (for example, advanced HIV or transplant recipients). In these settings, resistant virus can be selected for over time [1][2].
What might resistance look like clinically?
Resistance is usually suspected when herpes infections do not improve as expected despite adequate dosing, or they recur quickly during therapy. Clinicians typically confirm suspected resistance with viral testing when available, since symptoms alone can overlap with other causes of treatment failure [1].
What are the main alternatives if resistance is suspected or confirmed?
If resistance is suspected, clinicians may switch to other antivirals that have different activation pathways or activity against resistant strains. Choices depend on the virus type and resistance pattern, and may include drugs such as foscarnet or cidofovir in more complex or refractory cases [1][2].
Does resistance mean acyclovir will never work for that person again?
Not necessarily. Resistance is about the specific viral population present. If resistant virus emerges, subsequent episodes can be harder to control with acyclovir, but treatment decisions are guided by testing and the clinical course rather than assuming total loss of activity [1][2].
Sources
- UpToDate: Antiviral drugs for herpes simplex virus infection (acyclovir resistance mechanisms and clinical considerations)
- NCBI Bookshelf: Acyclovir resistance (mechanisms and risk factors)