Inclusion Criteria:

- Age ≥18 years of age

- Receiving intravenous acyclovir 5 mg/kg (total body weight [TBW] for normal weight
patients and ideal body weight [IBW] for obese patients) as part of their routine
care

- Admitted as an inpatient with an expected stay of at least 24 hours

- Weight > 190% of ideal body weight (IBW) for "obese" patients or weight 80-120% of
IBW for matched control patients.

Exclusion Criteria:

- Receipt of acyclovir or similar agents (valacyclovir, ganciclovir, valganciclovir,
famciclovir) in the prior 24 hours

- Serum creatinine > 1.5 mg/dL at time of drug administration

- Hypersensitivity to acyclovir

- Patients requiring ventilator support or vasopressors in the prior 24 hours

- Receipt of probenecid, mycophenolate, tenofovir, or zidovudine in the prior 7 days

- Pregnant or breast-feeding

- Significant anatomical deformities that influence body habitus (i.e. amputation)

- Prior inclusion in this study