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Prolonged use of acyclovir, an antiviral medication commonly used to treat herpes simplex virus and varicella-zoster virus infections, has been associated with potential kidney function impairment. According to the U.S. National Library of Medicine's Drug Information Portal, acyclovir is primarily excreted by the kidneys, which can lead to kidney-related side effects [1].

Acyclovir's kidney function impact is often dose-dependent and more prevalent in patients with pre-existing kidney issues or those receiving higher doses [1]. The most common kidney-related side effect of acyclovir is elevated creatinine levels, which can indicate decreased kidney function [1]. In rare cases, acyclovir use can lead to more severe kidney complications, such as acute kidney injury or failure [1].

A study published in the Journal of Clinical Pharmacy and Therapeutics found that prolonged acyclovir use in renal transplant recipients was associated with a higher risk of nephrotoxicity [2]. However, the study did not establish a causal relationship between acyclovir and long-term kidney function impairment [2].

It is essential to monitor kidney function during prolonged acyclovir treatment, particularly in patients with pre-existing kidney issues or those receiving higher doses [1]. Dosage adjustments may be necessary to minimize the risk of kidney-related side effects [1].

In summary, prolonged acyclovir use may impact long-term kidney function, particularly in patients with pre-existing kidney issues or those receiving higher doses. Close monitoring of kidney function during treatment is recommended to minimize potential kidney-related side effects [1, 2].

Sources:
[1] U.S. National Library of Medicine. (2021). Acyclovir - Drug Information Portal. Retrieved from <https://druginfo.nlm.nih.gov/drugportal/name/acyclovir>.
[2] Bailie, G. R., & Gallagher, R. M. (1995). Nephrotoxicity of antiviral agents in renal transplant recipients. Journal of Clinical Pharmacy and Therapeutics, 20(3), 169-173. doi: 10.1111/j.1365-2710.1995.tb00121.x (<https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2710.1995.tb00121.x>)

(Note: DrugPatentWatch.com was not used as a source for this answer.)