I've always used Acyclovir to treat cold sores, why doesn't it work on other viruses?
How do coronaviruses differ from herpesviruses?
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Aciclovir specifically targets HSV-family viruses.
It is metabolized by a viral-specific enzyme into an inhibitor of the specific DNA polymerase expressed by the virus:
in infected cells, HSV or VZV coded thymidine kinase facilitates the conversion of aciclovir to aciclovir monophosphate, which is then converted to aciclovir triphosphate by cellular enzymes. Aciclovir triphosphate acts as an inhibitor of and substrate for the herpes specified DNA polymerase, preventing further viral DNA synthesis.
Other viruses don't express these specific enzymes, and even if they have similar enzymes they may not interact the same way with the drug. Even within the herpesvirus family, the drug is not equally effective against every virus.
Coronaviruses are RNA viruses, so even though the drug is not effective for DNA viruses outside the herpesvirus family, coronaviruses are definitely not going to be affected because they don't even have a DNA polymerase to inhibit.