What is Bactrim (trimethoprim-sulfamethoxazole) SS (single-strength)?

Bactrim SS (Single-Strength): Composition and Clinical Applications

Bactrim SS (single-strength) is an oral antibacterial tablet containing 80 mg trimethoprim and 400 mg sulfamethoxazole, which is half the dose of a double-strength tablet. 1

Composition and Mechanism of Action

Bactrim (trimethoprim-sulfamethoxazole or TMP-SMZ) is a synthetic antibacterial combination product that works through a synergistic mechanism:

• Sulfamethoxazole: Inhibits bacterial synthesis of dihydrofolic acid by competing with para-aminobenzoic acid (PABA)
• Trimethoprim: Blocks the production of tetrahydrofolic acid by inhibiting the enzyme dihydrofolate reductase

This dual mechanism blocks two consecutive steps in bacterial folate synthesis, which is essential for bacterial nucleic acid and protein production. 1

Formulations Available

• Single-strength (SS): 80 mg trimethoprim + 400 mg sulfamethoxazole
• Double-strength (DS): 160 mg trimethoprim + 800 mg sulfamethoxazole 1

Clinical Applications

HIV-Related Prophylaxis

• For Pneumocystis carinii pneumonia (PCP) prophylaxis in HIV patients:
  • One single-strength tablet daily is effective and may be better tolerated than double-strength tablets 2
  • Provides cross-protection against toxoplasmosis and common respiratory bacterial infections 2

Urinary Tract Infections

• Effective for acute uncomplicated urinary tract infections
• Single-strength dosing may be appropriate for uncomplicated UTIs 3, 4

Other Infections

• Effective against various respiratory tract infections
• Used for skin and soft tissue infections
• Active against many common pathogens including E. coli, Klebsiella species, Enterobacter species, Morganella morganii, and Proteus species 1

Pharmacokinetics

• Mean serum half-lives: 10 hours for sulfamethoxazole and 8-10 hours for trimethoprim
• Both components are primarily excreted by the kidneys
• Dosage adjustment required in patients with severely impaired renal function 1
• Geriatric patients may have lower trimethoprim clearance (approximately 19% lower than young adults) 1

Adverse Effects

• Generally well-tolerated at single-strength dosing 3
• Common side effects include gastrointestinal disturbances and hypersensitivity skin reactions
• Serious but rare adverse effects include Stevens-Johnson syndrome, blood dyscrasias, and hepatic necrosis 5
• Adverse reactions occur less frequently with trimethoprim alone than with the combination product 3

Special Considerations

• Renal Impairment: Dosage adjustment required when creatinine clearance is less than 30 mL/min 6
• Elderly Patients: May require dose adjustment due to reduced clearance and higher peak concentrations 5
• Monitoring: Complete blood counts recommended at initiation and monthly intervals to assess for hematologic toxicity 5

Practical Prescribing Tips

• For patients experiencing non-life-threatening adverse reactions, consider gradual reintroduction of the drug (desensitization) or using a reduced dose or frequency 2
• Up to 70% of patients can tolerate reinstitution of therapy after an adverse event has resolved 2

Bactrim SS provides a lower dose option that maintains efficacy while potentially reducing the risk of adverse effects, making it particularly useful for long-term prophylaxis or in patients who cannot tolerate the double-strength formulation.