Acyclovir and valacyclovir are acyclic guanosine derivatives, with clinical activity against herpes simplex virus 1 and 2, and varicella zoster virus. Both drugs are tolerated well and exhibit a favourable toxicity profile. However, in patients with impaired creatinine clearance, dose adjustments are mandated for both drugs for the purpose of patient safety, without compromising the clinical efficacy of these drugs.