This section provides information on the proper use of a number of products that contain sulfamethoxazole and trimethoprim.  It may not be specific to Bactrim DS. Please read with care.
Take this medicine exactly as directed by your doctor. Do not take more of it, do not take it more often, and do not take it for a longer time than your doctor ordered. To do so may increase the chance of side effects.
Sulfamethoxazole and trimethoprim combination is best taken with a full glass (8 ounces) of water. Several additional glasses of water should be taken every day, unless otherwise directed by your doctor. Drinking extra water will help to prevent some unwanted effects. .
For patients taking the oral liquid, use a specially marked measuring spoon or other device to measure each dose accurately. The average household teaspoon may not hold the right amount of liquid.
To help clear up your infection completely, keep using this medicine for the full time of treatment, even if you begin to feel better after a few days. If you stop taking this medicine too soon, your symptoms may return.
Dosing
The dose of this medicine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of this medicine. If your dose is different, do not change it unless your doctor tells you to do so.
The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.
For oral dosage forms (liquid or tablets):
For treatment of bacterial infections:
Adults—1 tablet (DS tablet) of 800 milligrams (mg) of sulfamethoxazole and 160 mg of trimethoprim, 2 tablets of 400 mg of sulfamethoxazole and 80 mg of trimethoprim, or 4 teaspoonfuls or 20 milliliters (mL) of oral liquid every 12 hours for 10 to 14 days. Your doctor may adjust this dose if needed.
Children 2 months of age and older—Dose is based on body weight and must be determined by your doctor. The usual dose is 40 milligrams (mg) per kilogram of body weight of sulfamethoxazole and 8 milligrams (mg) per kilogram of body weight of trimethoprim, given in two divided doses every 12 hours for 10 days.
Weighing 40 kilograms (kg)—4 teaspoonfuls (20 mL) of oral liquid or 2 tablets (or 1 DS tablet) 2 times a day for 10 days.
Weighing 30 kg—3 teaspoonfuls (15 mL) of oral liquid or 1 ½ tablet 2 times a day for 10 days.
Weighing 20 kg—2 teaspoonfuls (10 mL) of oral liquid or 1 tablet 2 times a day for 10 days.
Weighing 10 kg—1 teaspoonful (5 mL) of oral liquid 2 times a day for 10 days.
Children younger than 2 months of age—Use is not recommended.
For treatment of bronchitis:
Adults—1 tablet (DS tablet) of 800 milligrams (mg) of sulfamethoxazole and 160 mg of trimethoprim, 2 tablets of 400 mg of sulfamethoxazole and 80 mg of trimethoprim, or 4 teaspoonfuls or 20 milliliters (mL) of oral liquid every 12 hours for 14 days.
Children 2 months of age and older—Use and dose must be determined by your doctor.
Children younger than 2 months of age—Use is not recommended.
For treatment of Pneumocystis jiroveci pneumonia or Pneumocystis carinii pneumonia (PCP):
Adults and children 2 months of age and older—Dose is based on body weight and must be determined by your doctor. The usual dose is 75 to 100 milligrams (mg) per kilogram of body weight of sulfamethoxazole and 15 to 20 milligrams (mg) per kilogram of body weight of trimethoprim each day, given in equally divided doses every 6 hours for 14 to 21 days.
Weighing 80 kilograms (kg)—10 teaspoonfuls (50 milliliters (mL)) or 5 tablets (or 2 ½ DS tablets) 4 times a day for 14 to 21 days.
Weighing 64 kg—8 teaspoonfuls (40 mL) or 4 tablets (or 2 DS tablets) 4 times a day for 14 to 21 days.
Weighing 48 kg—6 teaspoonfuls (30 mL) or 3 tablets (or 1 ½ DS tablets) 4 times a day for 14 to 21 days.
Weighing 40 kg—5 teaspoonfuls (25 mL) or 2 ½ tablets 4 times a day for 14 to 21 days.
Weighing 32 kg—4 teaspoonfuls (20 mL) or 2 tablets (or 1 DS tablet) 4 times a day for 14 to 21 days.
Weighing 24 kg—3 teaspoonfuls (15 mL) or 1 ½ tablets 4 times a day for 14 to 21 days.
Weighing 16 kg—2 teaspoonfuls (10 mL) or 1 tablet 4 times a day for 14 to 21 days.
Weighing 8 kg—1 teaspoonful (5 mL) 4 times a day for 14 to 21 days.
Children younger than 2 months of age—Use is not recommended.
For prevention of Pneumocystis jiroveci pneumonia or Pneumocystis carinii pneumonia (PCP):
Adults—800 milligrams (mg) of sulfamethoxazole and 160 mg of trimethoprim once a day.
Children 2 months of age and older—Dose is based on body size and must be determined by your doctor. The usual dose is 750 mg of sulfamethoxazole and 150 mg of trimethoprim per square meter (m[2]) of body surface each day. This is given in equally divided doses two times a day for 3 days a week on consecutive days (eg, Monday, Tuesday, Wednesday). However, the dose is usually not more than 1600 mg of sulfamethoxazole and 320 mg of trimethoprim per day.
Children younger than 2 months of age—Use is not recommended.
For treatment of traveler's diarrhea:
Adults—1 tablet (DS tablet) of 800 milligrams (mg) of sulfamethoxazole and 160 mg of trimethoprim, 2 tablets of 400 mg of sulfamethoxazole and 80 mg of trimethoprim, or 4 teaspoonfuls or 20 milliliters (mL) of oral liquid every 12 hours for 5 days.
Children 2 months of age and older—Use and dose must be determined by your doctor.
Children younger than 2 months of age—Use is not recommended.
Missed dose
If you miss a dose of this medicine, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not double doses.
Storage
Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light. Keep from freezing.
Keep out of the reach of children.
Do not keep outdated medicine or medicine no longer needed.
Ask your healthcare professional how you should dispose of any medicine you do not use.
 Detailed Bactrim DS dosage information
