Acyclovir is generally considered safe for long-term use, but like any medication, it does carry some risks. Common side effects of acyclovir include nausea, vomiting, diarrhea, and headache. More serious side effects are rare but can include kidney problems, allergic reactions, and changes in blood cell counts. It is important to consult with a healthcare provider before using acyclovir long term, especially for individuals with kidney issues or compromised immune systems. Monitoring by a healthcare professional can help mitigate any potential risks associated with prolonged acyclovir use [3].

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/ACYCLOVIR
[2] https://pubmed.ncbi.nlm.nih.gov/2895840/
[3] https://www.drugs.com/sfx/acyclovir-side-effects.html