Bactrim Dosage


Bactrim (a combination of two antibiotics: sulfamethoxazole and trimethoprim) is a medication that’s used to treat an expansive list of bacterial infections, such as staph/MRSA. While Bactrim is safely prescribed for patients 2 months of age and older, the dosage needs to be carefully followed to ensure it’s long-term effectiveness for the patients. Below we’ve presented a helpful table for you to cross-reference dosage levels for your patients. Please note that administering Bactrim to newborns or infants younger than 2 months old is not advised due to the risk of serious side effects.

Trimethoprim/sulfamethoxazole Suspension
(40/200 per 5mL)

(must be > 2 months old)

WEIGHTDOSE (Infection*)
(lbs)(kg)(10mg TMP/kg/day)
22 lbs10 kg1 tsp PO BID
44 lbs20 kg2 tsp BID or 1 tab BID
66 lbs30 kg3 tsp BID or 1.5 tab BID
88 lbs40 kg4 tsp BID or 2 tab BID or 1 DS tab BID
Adult (skin infection; MRSA)1 DS tab PO BID ** (others will give 2 DS tab PO BID to obese patients)

*doses are for acute otitis media or UTI
**reference: personal communication with Dirk Elston, MD
(1 tsp = 1 teaspoon = 5mL)
(TMP = trimethoprim)
1 tab = 80/400 (= 2 tsp of the suspension)
1 DS tab = 160/800 (= 4 tsp of the suspension)


High Dose Bactrim for treating PCP (as a point of reference)

WEIGHTDOSE – Every 6 Hours (= 20mg TMP/kg/day dose)
18 lbs8 kg1 (5 mL)
35 lbs16 kg2 (10 mL)1
53 lbs24 kg3 (15 mL)1 1/2
70 lbs32 kg4 (20 mL)2 (or 1 DS Tablet)
88 lbs40 kg5 (25 mL)2 1/2
106 lbs48 kg6 (30 mL)3 (or 1 1/2 DS Tablets)
141 lbs64 kg8 (40 mL)4 (or 2 DS Tablets)
176 lbs80 kg10 (50 mL)5 (or 2 1/2 DS Tablets)

For the lower limit dose (15 mg/kg trimethoprim and 75 mg/kg sulfamethoxazole per 24 hours) administer 75% of the dose in the above table.

Disclaimer: All of the information on this website is being presented as helpful reference material but should not be considered authoritative. There are no references because the information has been collected over the years piecemeal from notes, lectures, journals, textbooks, personal communications with other physicians and experience. You may disagree with some of it. Take it simply for what it is: information that I have found useful in my day to day practice as a clinical dermatologist and my hope is that you find it useful as well.

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