What is the standard Bactrim dose for a UTI?

For adults, the standard Bactrim dose for an uncomplicated UTI is 1 double-strength (DS) tablet (160mg TMP / 800mg SMX) twice daily for 3 days (uncomplicated cystitis) or 7–14 days (complicated UTI or pyelonephritis). For children, the dose is 8–10 mg TMP/kg/day divided into two doses (BID). TMP-SMX is a first-line agent for uncomplicated UTI when local resistance rates for E. coli are below 20%.

How is TMP-SMX dosing calculated for children?

Pediatric TMP-SMX dosing is based on the trimethoprim (TMP) component per kilogram of body weight. Multiply the child's weight in kg by the recommended mg TMP/kg/day for the specific indication, then divide by the number of daily doses. For example, a 20 kg child with a UTI: 8 mg/kg/day × 20 kg = 160 mg TMP/day ÷ 2 doses = 80 mg TMP per dose. Using the suspension (40 mg TMP per 5 mL), this equals 10 mL per dose.

What is the Bactrim dose for PJP treatment?

For Pneumocystis jirovecii pneumonia (PJP) treatment, the dose is 15–20 mg TMP/kg/day divided every 6–8 hours for 21 days. This is significantly higher than standard dosing. For a 70 kg adult, this equals 1050–1400 mg TMP/day (approximately 7–9 DS tablets/day) or IV dosing. PJP prophylaxis uses a much lower dose: 1 DS tablet daily or 1 DS tablet three times per week.

Does Bactrim need renal dose adjustment?

Yes. For patients with CrCl 15–30 mL/min, reduce the dose by 50%. For CrCl below 15 mL/min, TMP-SMX should generally be avoided due to the risk of accumulation and hyperkalemia. TMP itself can cause a transient rise in serum creatinine by inhibiting tubular secretion (not a true change in GFR). Monitor potassium and renal function closely, especially when combined with ACE inhibitors or ARBs.

What are the main side effects and drug interactions of Bactrim?

Common side effects include nausea, vomiting, rash, and photosensitivity. Serious risks include hyperkalemia (TMP blocks ENaC channels), bone marrow suppression (folate antagonism — monitor CBC with prolonged use), Stevens-Johnson syndrome, and hemolytic anemia in G6PD-deficient patients. Key drug interactions include warfarin (TMP-SMX increases INR significantly), methotrexate (increased toxicity via folate pathway competition), and phenytoin (increased phenytoin levels). Avoid in pregnancy, particularly in the first trimester (folate antagonism) and near term (kernicterus risk).

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Bactrim (TMP-SMX) Dosage Calculator

Calculate trimethoprim-sulfamethoxazole dose for adults and children by weight and indication. Shows TMP and SMX components, formulation amounts for suspension, SS/DS tablets, and IV with dilution guidance.

Inputs

Patient type

Weight unit

Patient weight lbs

Indication

Preferred formulation

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Enter weight and select indication to calculate dose.

Disclaimer: For educational purposes only. Not a substitute for clinical judgment. Always verify doses with current guidelines and pharmacy references.

Dosing reference (TMP component)

UTI (standard): 8–10 mg/kg/day ÷ BID; Adult: 1 DS tab BID
UTI prophylaxis: 2 mg/kg/day once daily
Otitis media: 8 mg/kg/day ÷ BID
MRSA skin/soft tissue: 8–12 mg/kg/day ÷ BID; Adult: 1–2 DS tabs BID
PJP treatment: 15–20 mg/kg/day ÷ q6-8h (21 days)
PJP prophylaxis: 5 mg/kg/day ÷ BID or 1 DS daily / 3×/week
Shigella / travelers' diarrhea: 8 mg/kg/day ÷ BID
Stenotrophomonas: 15 mg/kg/day ÷ q6-8h

Renal dose adjustment

CrCl >30 mL/min: No adjustment
CrCl 15–30 mL/min: 50% of normal dose
CrCl <15 mL/min: Avoid use

Clinical warnings

Hyperkalemia: TMP blocks ENaC channels → monitor K⁺, especially with ACE inhibitors/ARBs.
Folate antagonism: Monitor CBC with prolonged use (>14 days). Consider leucovorin in high-dose regimens.
G6PD deficiency: Risk of hemolytic anemia — screen before use when possible.
Pregnancy: Avoid in 1st trimester (folate antagonism → NTD risk) and near term (3rd trimester: kernicterus risk).
Drug interactions: Warfarin (↑INR — monitor closely), methotrexate (↑toxicity), phenytoin (↑levels).
Creatinine artifact: TMP inhibits tubular creatinine secretion → SCr may rise 10–20% without true GFR change.

Available formulations

Suspension: 40 mg TMP / 200 mg SMX per 5 mL
SS tablet: 80 mg TMP / 400 mg SMX
DS tablet: 160 mg TMP / 800 mg SMX
IV: 16 mg TMP / 80 mg SMX per mL

References

Lexicomp / UpToDate Drug Information: Trimethoprim-Sulfamethoxazole.
Liu C, et al. IDSA Clinical Practice Guidelines for MRSA Infections. Clin Infect Dis. 2011;52(3):e18–e55.
HHS/IDSA/CDC Guidelines for Prevention and Treatment of Opportunistic Infections (PJP). 2024.
Bactrim (trimethoprim-sulfamethoxazole) Prescribing Information. Roche/Sun Pharma.

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About This Tool

What Is Bactrim (TMP-SMX)?

Bactrim is a brand name for the combination antibiotic trimethoprim-sulfamethoxazole (TMP-SMX), also known as co-trimoxazole. It works through sequential blockade of folate synthesis: sulfamethoxazole inhibits dihydropteroate synthase while trimethoprim inhibits dihydrofolate reductase. This dual mechanism produces synergistic bactericidal activity against a broad spectrum of gram-positive and gram-negative organisms. TMP-SMX is available as oral suspension, single-strength (SS) tablets, double-strength (DS) tablets, and intravenous formulation.

How TMP-SMX Dosing Works

All TMP-SMX dosing is based on the trimethoprim (TMP) component. The fixed ratio of TMP to SMX is always 1:5 (e.g., 160 mg TMP paired with 800 mg SMX in a DS tablet). When a dose is expressed as "15 mg/kg/day," this refers to 15 mg of TMP per kilogram per day. The SMX component is automatically determined by the 1:5 ratio. Weight-based dosing is used for most indications, especially in pediatrics, while adult dosing often defaults to standard tablet-based regimens (e.g., 1 DS tablet BID for UTI).

Bactrim Dosage by Indication

Urinary tract infections: The most common use. Adults take 1 DS tablet BID (160 mg TMP BID). Pediatric dose is 8–10 mg TMP/kg/day divided BID. Duration: 3 days for uncomplicated cystitis, 7–14 days for complicated UTI. Consider local E. coli resistance patterns — TMP-SMX should not be used empirically if local resistance exceeds 20%.

MRSA skin and soft tissue infections: TMP-SMX is a first-line oral agent for outpatient MRSA infections per IDSA guidelines. Adults: 1–2 DS tablets BID. Pediatric: 8–12 mg TMP/kg/day divided BID. Duration typically 7–10 days for uncomplicated skin infections.

PJP treatment: The highest dose indication at 15–20 mg TMP/kg/day divided every 6–8 hours for 21 days. This often requires IV formulation initially. For a 70 kg patient, this is 1050–1400 mg TMP/day. Adjunctive corticosteroids recommended if PaO₂ <70 mmHg or A-a gradient >35.

PJP prophylaxis: 1 DS tablet daily or 1 DS tablet three times per week. Pediatric: 5 mg TMP/kg/day divided BID (max 320 mg TMP/day). Indicated for HIV patients with CD4 <200 or CD4% <14%, and other immunosuppressed patients.

🔑 Clinical Pearls

TMP-SMX has excellent bioavailability (~95%) — IV-to-oral switch can often occur early when patient is tolerating PO.
The creatinine rise from TMP (blocks tubular secretion) is predictable and benign — don't reflexively stop the drug. True nephrotoxicity is rare.
For MRSA, TMP-SMX does NOT cover group A streptococcus — if concerned about GAS co-infection (e.g., cellulitis), add a beta-lactam.
Liquid formulation contains sorbitol and may cause osmotic diarrhea at high volumes — consider tablets crushed in food for older children at PJP treatment doses.
Desensitization protocols exist for sulfa-allergic patients who require PJP prophylaxis (common in HIV care).
Photosensitivity is common — counsel on sun protection during treatment.

Key References

Liu C, et al. Clinical Practice Guidelines by IDSA for the Treatment of MRSA Infections in Adults and Children. Clin Infect Dis. 2011;52(3):e18–e55.
Panel on Opportunistic Infections in Adults and Adolescents with HIV. Guidelines for the Prevention and Treatment of Opportunistic Infections. AIDSinfo/HHS. 2024.
Gupta K, et al. International Clinical Practice Guidelines for the Treatment of Acute Uncomplicated Cystitis and Pyelonephritis. Clin Infect Dis. 2011;52(5):e103–e120.
Bactrim (trimethoprim and sulfamethoxazole) Prescribing Information. Roche/Sun Pharmaceutical.

Formula last verified: February 2026