Patients receiving acyclovir as standard of care will be enrolled into this study. The currently recommended dosing strategy at our institution for obese patients is to receive intravenous acyclovir dosed per an adjusted body weight [IBW + 0.4(TBW-IBW)]. They will have blood drawn once prior to the first dose of acyclovir and 10 times thereafter, over a total time period of 12 hours. These patients will already be hospitalized for other reasons, and will not be required to make additional trips to the hospital. A total of approximately 4-5 tablespoons of blood will be drawn for this study. Ten obese patients and 10 matched control (non-obese) patients will be enrolled.

Inclusion Criteria:

• Age ≥18 years of age
• Receiving intravenous acyclovir 5 mg/kg (TBW for normal weight patients and ABW40 for obese patients) as part of their routine care
• 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 a pro-drug of acyclovir (valacyclovir, ganciclovir, valganciclovir, famciclovir) in the prior 24 hours
• Serum creatinine >1.5 mg/dL
• 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
• Significant anatomical deformities that influence body habitus (i.e. amputation)
• Prior inclusion in this study

Obese and non-obese patients receiving intravenous acyclovir