Acyclovir is an antiviral drug that is used to treat infections caused by herpes viruses, including genital herpes, cold sores, shingles, and chickenpox. Long-term use of acyclovir has been studied in relation to its effect on immunity.

According to Drug Patent Watch [1], acyclovir is generally well-tolerated and does not appear to have a significant impact on the immune system. However, long-term use of acyclovir may lead to the development of drug-resistant strains of herpes viruses, which could potentially make it more difficult to treat future outbreaks.

A study published in the Journal of Infectious Diseases [2] found that long-term acyclovir therapy did not appear to have a significant impact on the immune system of patients with herpes simplex virus infections. The study followed patients for up to 10 years and found no evidence of increased susceptibility to other infections or decreased immune function.

Another study published in the Journal of Clinical Virology [3] also found that long-term acyclovir therapy did not appear to have a negative impact on the immune system. The study followed patients with recurrent genital herpes who were treated with acyclovir for up to 5 years and found no evidence of increased susceptibility to other infections or decreased immune function.

In conclusion, the available evidence suggests that long-term use of acyclovir does not have a significant impact on the immune system. However, there is a risk of developing drug-resistant strains of herpes viruses with long-term use, which could potentially make it more difficult to treat future outbreaks. Patients should discuss the risks and benefits of long-term acyclovir therapy with their healthcare provider.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/ACYCLOVIR
[2] https://pubmed.ncbi.nlm.nih.gov/3183633/
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4414608/