Acyclovir is an antiviral medication that is commonly used to treat herpes simplex virus infections, varicella-zoster virus infections, and other viral infections. There is limited research on how acyclovir affects cancer risk. However, some studies suggest that acyclovir may have a protective effect against certain types of cancer.

One study published in Infectious Agents and Cancer [2] suggested that acyclovir may reduce the risk of certain cancers, including lung cancer, colorectal cancer, and prostate cancer. The study found that patients who took acyclovir had a lower risk of developing these types of cancer compared to patients who did not take the medication.

Another study published in the Journal of the National Cancer Institute [3] found that acyclovir may reduce the risk of developing cervical cancer in women with herpes simplex virus infections. The study found that women who took acyclovir had a lower risk of developing cervical cancer compared to women who did not take the medication.

Despite these findings, it is important to note that more research is needed to fully understand the relationship between acyclovir and cancer risk. Additionally, acyclovir is not recommended as a cancer prevention medication and should only be used as directed by a healthcare provider for the treatment of viral infections.

Sources:
[2] https://infectagentscancer.biomedcentral.com/articles/10.1186/s13027-017-0128-7
[3] https://academic.oup.com/jnci/article/94/21/1604/2519467
[1] https://www.webmd.com/drugs/2/drug-941/acyclovir-oral/details