This study will find how weight affects the dosing of a drug called sulfamethoxazole and trimethoprim. Currently, the amount of sulfamethoxazole and trimethoprim a patient receives is the same regardless of the patient's weight. The entire cohort was analyzed for the study outcomes. BMI groups were for recruitment purposes only and were not used for ordinal data analysis.

All sulfamethoxazole and trimethoprim (Trade name is Bactrim or Septra) medication that you will receive in this study will be referred to as study medication within this informed consent form. This drug is a combination of two antibiotics, sulfamethoxazole and trimethoprim, which belongs to a class of medication known as "sulfones" and is approved by the US Food and Drug Administration (FDA) for the treatment of a wide variety of bacterial infections such as ear infections, urinary tract infections, bronchitis, traveler's diarrhea, and Pneumocystis carinii pneumonia. Sulfamethoxazole and trimethoprim is given orally.

This study is designed to measure drug concentrations in the blood of volunteers administered a single oral dose of sulfamethoxazole and trimethoprim. The volunteers to be enrolled will not have ear infections, urinary tract infections, bronchitis, traveler's diarrhea, Pneumocystis carinii pneumonia, or any other bacterial infection. This is a single center study. A total of 36 adult volunteers will be consented for the study at the Clinical and Translational Research Center (CTRC). Volunteers will be recruited using IRB approved fliers. Volunteers will be admitted to the CTRC for an overnight stay. Half will be female and half male. Twelve volunteers will have a body mass index (BMI) less than 25 kg/m2, twelve will have a BMI 25-40 kg/m2, and twelve will have a BMI greater than 40 kg/m2. Volunteers will have height and weight measured after they have consented to participate. All volunteers will receive a single oral dose of sulfamethoxazole and trimethoprim of 1600 mg/320 mg. The volunteers will have blood drawn via an intravenous catheter just prior to the dose, and then at 1, 2, 4, 8, 12, and 24h after the drug dose. The intravenous catheter is then removed after the 24h blood draw, and the volunteer is discharged from the study.

Inclusion Criteria:

• Male and female subjects, age >18 years, of all racial and ethnic origins.
• We are recruiting 12 normal or underweight (BMI <25kg/m2), 12 overweight or obese (BMI 25-40 kg/m2), and 12 extremely obese (BMI > 40 kg/m2) for this study. This index is calculated using the volunteer's height and weight (Formula: weight (lb) / [height (in)]2 x 703). Half of each group will be male; the other half will be female.

Exclusion Criteria:

• Pregnant or nursing or unwilling to use a reliable contraception method during the study. The effects of sulfamethoxazole and trimethoprim on pregnancy are unknown. In addition, the metabolic changes that accompany pregnancy may alter the concentration-time profile of sulfamethoxazole and trimethoprim, so that the pregnancy and post-partum state would be a confounding variable.
• Abnormal liver function tests: transaminases>10 times upper limit of normal, Alkaline phosphatase>5 times upper limit of normal, total bilirubin>5 times upper limit of normal.
• History of allergies to sulfones, sulfonamides or trimethoprim.
• Sulfones, sulfonamides or trimethoprim are contraindicated for any reason.
• Volunteers unwilling to comply with study procedures.
• Current suspected or documented ear infection, urinary tract infection, bronchitis, traveler's diarrhea, Pneumocystis carinii pneumonia, or any other bacterial infection.
• Volunteers with colon resection, gastric bypass, lap band, or any other conditions inhibiting gastric absorption of drug.
• Current or previous participation within 28 days of enrollment in another research study that involves the use of medication, contrast, or any other compound that may alter blood count and/or blood chemistry (liver function, kidney function or electrolyte balance), unless waved by PI.
• Donation of 450mL (one unit) of blood or more within 8 weeks (56 days) prior to study enrollment, unless waved by PI.
• Current use of medications contraindication with sulfamethoxazole/trimethoprim use: Bepridil, Cisapride, Dofetilide, Levomethadyl, Mesoridazine, Pimozide, Terfenadine, and Thioridazine. Other medications will be screened by study investigators to ensure the safety of research participants and maintain the quality of the study.

24 hours