Studies have shown that different percentages of body fat can alter the way drugs are distributed in the body. This study will use blood samples taken at different time points for patients taking acyclovir to determine if higher body weights affect drug exposure. The information gathered from this study will help understand if patients with higher body weights need a different dosing plan.

Patients receiving acyclovir as standard of care will be enrolled into this study. They will have blood draws once before they take acyclovir and 10 times after they take the acyclovir (over a total of 12 hours). These patients will be in the hospital already and will not need to make additional trips back to have blood drawn. A total of about 4-5 tablespoons of blood will be drawn for this study. 7 obese patients and 7 matched, non-obese patients will be enrolled into this study.

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

Obese and normal weight patients treated with acyclovir in an inpatient setting.