A white oval pill debossed with H 49 is a generic prescription formulation of Sulfamethoxazole and Trimethoprim Double Strength (800 mg / 160 mg). Commonly referred to by brand names such as Bactrim DS or Septra DS, it is a combination antibiotic from the sulfonamide class.
Pill Identification
Characteristic Specification
- Brand Equivalent – Bactrim DS, Septra DS, Co-trimoxazole
- Active Ingredients – Sulfamethoxazole (800 mg) + Trimethoprim (160 mg)
- Imprint Code –
H 49(one side) / Deep break line / score (other side) - Drug Class – Sulfonamides / Anti-infectives
- Color & Shape – White to off-white, Oval (Beveled Edge)
- Size – 19 mm
- Manufacturer / Labeler – Aurobindo Pharma USA
- Controlled Substance Class – Non-controlled (Rx required)
- Note: Pill markings and dimensions can differ among manufacturers. If you are ever unsure about a prescription tablet, confirm its identity with a licensed pharmacist before taking it.
Indications & Medical Uses
Sulfamethoxazole and Trimethoprim DS is indicated for treating susceptibility-tested bacterial infections:
- Urinary Tract Infections (UTIs): Uncomplicated and complicated cystitis or pyelonephritis caused by susceptible strains of E. coli, Klebsiella, Enterobacter, or Proteus species.
- Respiratory Infections: Acute infective exacerbations of chronic bronchitis, middle ear infections (acute otitis media in children), and bacterial sinus infections.
- Gastrointestinal Infections: Shigellosis (bacillary dysentery) and traveler’s diarrhea caused by susceptible enterotoxigenic E. coli.
- Pneumocystis Jirovecii Pneumonia (PJP/PCP): Prevention and treatment of PJP, a serious fungal/opportunistic lung infection common in immunocompromised patients (such as individuals living with HIV/AIDS or organ transplant recipients).
- Skin & Soft Tissue Infections: Off-label treatment for skin infections, including community-acquired Methicillin-Resistant Staphylococcus aureus (MRSA).
Mechanism of Action
Sulfamethoxazole and trimethoprim work together synergistically to block two sequential steps in bacterial folate synthesis, making the combination bactericidal (killing bacteria) rather than merely bacteriostatic (inhibiting growth):
- Step 1 (Sulfamethoxazole): Competes with para-aminobenzoic acid (PABA) to inhibit the bacterial enzyme dihydropteroate synthase, halting the early production of dihydrofolic acid.
- Step 2 (Trimethoprim): Reversibly inhibits dihydrofolate reductase (DHFR), stopping the conversion of dihydrofolic acid into tetrahydrofolic acid.
Because bacteria must synthesize their own folate to produce purines, pyrimidines, and essential amino acids for DNA replication, blocking both steps stops bacterial growth and survival without significantly disrupting human folate pathways at normal clinical doses.
Dosage & Administration
Dosage is tailored to the infection site, infection severity, renal function, and body weight (especially in pediatric patients).
- Standard Adult Dosage (e.g., UTIs, Bronchitis):
- 1 Double Strength tablet (800 mg/160 mg) every 12 hours for 3 to 14 days, depending on the severity of the infection.
- Severe Infections or PJP Treatment:
- Dosages are calculated based on trimethoprim content per kilogram of body weight per day (usually divided into doses every 6 hours).
- Renal Impairment Adjustments:
- Creatinine Clearance 15 to 30 mL/min: Dosage is reduced by half (50% of standard regimen).
- Creatinine Clearance less than 15 mL/min: Use is generally not recommended.
Administration Tips:
- Take with a full glass of water. Drink plenty of fluids throughout the day to lower the risk of kidney stones or crystalluria (crystal formation in urine).
- Can be taken with food if stomach upset occurs.
Side Effects
Common Side Effects
- Gastrointestinal: Nausea, vomiting, loss of appetite, or mild diarrhea.
- Dermatologic: Mild skin rash or itching.
- Neurologic: Mild headache or dizziness.
Severe / Rare Reactions (Seek Emergency Care Immediately)
- Severe Cutaneous Adverse Reactions (SCARs): Life-threatening skin conditions like Stevens-Johnson Syndrome (SJS) or Toxic Epidermal Necrolysis (TEN). Symptoms include unexplained fever, painful mouth sores, skin peeling, or spreading red/purple rashes.
- Hematologic Abnormalities: Agranulocytosis, aplastic anemia, leukopenia, or thrombocytopenia. Watch for fever, unusual bleeding, bruising, or sore throat.
- Electrolyte Imbalances: High potassium levels (hyperkalemia), which can cause irregular heart rhythms or muscle weakness.
- Hepatic Toxicity: Jaundice (yellowing of eyes/skin), dark urine, or severe right upper abdominal pain.
- Severe Diarrhea: Clostridioides difficile-associated diarrhea (CDAD), which can develop weeks after antibiotic treatment.
Precautions & Interactions
- Drug Interactions:
- Warfarin: Sulfamethoxazole can significantly enhance the blood-thinning effect of warfarin, dramatically increasing bleeding risk. Frequent INR monitoring is required.
- ACE Inhibitors / ARBs: Combining with blood pressure medications like lisinopril or losartan increases the risk of severe hyperkalemia.
- Methotrexate: Increases methotrexate blood levels, worsening bone marrow suppression risk.
- Oral Antidiabetics (Sulfonylureas): Increases hypoglycemic effects, risking low blood sugar levels.
- Pre-existing Conditions: Use with extreme caution in patients with asthma, severe allergies, G6PD deficiency (risk of hemolytic anemia), or thyroid disorders.
Warnings & Contraindications
WARNINGS & CONTRAINDICATIONS
- Hypersensitivity: Strictly contraindicated in anyone with a known history of severe allergic reactions to sulfonamides (“sulfa” drugs) or trimethoprim.
- Pregnancy & Nursing:
- Pregnancy (Category D): Avoid during the first trimester (due to neural tube defect risks from folate antagonism) and during the late third trimester / near term (risk of kernicterus, a form of brain damage in newborns caused by high bilirubin levels).
- Nursing: Contraindicated while breastfeeding infants under 2 months of age.
- Age Restrictions: Do not administer to pediatric patients under 2 months old.
- Megaloblastic Anemia: Contraindicated in patients with diagnosed folate-deficiency megaloblastic anemia. >