What are the effects of antiviral drugs?

Depends on the drug: Some antivirals have methods of action that are very specific for a viral enzyme or protein and not people. Many, however, have effects that also cause toxicity in mammalian cells since that is where the virus replicates. Even those that are fairly specific can have side effects. As an example, some HIV drugs like azt (zidovudine) inhibit viral reverse transcriptase that we do not have but are still toxic.