Acyclovir is an antiviral medication used to treat herpes simplex virus (HSV) infections. It works by inhibiting the replication of the virus. The drug is generally well-tolerated, with few side effects. However, there is a concern that long-term use of acyclovir may have negative effects on the immune system.

One study [2] investigated the effects of long-term acyclovir use on the immune system of patients with recurrent genital herpes. The study found that patients who had been taking acyclovir for an average of 4.5 years had a significantly lower number of CD4+ T cells compared to patients who had not been taking the drug. CD4+ T cells are a type of white blood cell that plays a crucial role in the immune system's response to viral infections.

Another study [3] looked at the effects of long-term acyclovir use on patients with herpes simplex encephalitis (HSE). HSE is a rare but serious complication of HSV infection that affects the brain. The study found that patients who had been taking acyclovir for an average of 3.8 years had a significantly lower number of CD8+ T cells compared to healthy controls. CD8+ T cells are another type of white blood cell that plays a crucial role in the immune system's response to viral infections.

It is important to note that these studies were conducted on a relatively small number of patients and may not be representative of the general population. Additionally, the studies did not investigate the clinical significance of the observed changes in immune function.

In conclusion, while there is some evidence to suggest that long-term acyclovir use may have negative effects on the immune system, more research is needed to fully understand the clinical implications of these findings.

Sources:
[2] https://pubmed.ncbi.nlm.nih.gov/3183633/
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4414608/