Notes on Cotrimoxazole Pharmacology and MCQ for BPharm, GPAT, NEETPG, NORCET Exam
Cotrimoxazole Overview
Cotrimoxazole is a combination antibiotic medication that contains two active ingredients: sulfamethoxazole and trimethoprim. It’s commonly used to treat a variety of bacterial infections, such as urinary tract infections, respiratory infections (like bronchitis or pneumonia), and certain types of diarrhea caused by bacteria. It works by inhibiting bacterial growth—sulfamethoxazole blocks the production of folic acid in bacteria, while trimethoprim interferes with the bacteria’s ability to use folic acid, effectively starving them of a key nutrient they need to survive.
It’s often prescribed under brand names like Bactrim or Septra, but it’s also available as a generic drug. Doctors might choose it for its broad-spectrum activity, meaning it can target a wide range of bacteria. However, it’s not effective against viral infections, and like all antibiotics, it can have side effects—think nausea, rash, or, in rare cases, more serious reactions like kidney issues or allergic responses. If you’re asking for personal use, it’s worth checking with a healthcare provider, as its suitability depends on the specific infection and patient factors like allergies or resistance patterns.
Below are the notes on Cotrimoxazole
Cotrimoxazole is a fixed-dose combination of two antibiotics:
Sulfamethoxazole (SMX) — a sulfonamide Trimethoprim (TMP) — a dihydrofolate reductase inhibitor
Mechanism of Action
Cotrimoxazole works by inhibiting bacterial folic acid synthesis at two sequential steps:
| Drug | Target Enzyme |
|---|---|
| Sulfamethoxazole | Inhibits dihydropteroate synthase |
| Trimethoprim | Inhibits dihydrofolate reductase |
Result: Bactericidal synergy
Trimethoprim (Alone)
Sometimes used solo for UTIs or prostatitis
-
Bacteriostatic alone
-
Bactericidal when combined with SMX
Cotrimoxazole – Comparison Chart
| Aspect | Cotrimoxazole | Trimethoprim |
|---|---|---|
| Uses | – UTI, RTI (e.g., bronchitis, pneumonia) | – Uncomplicated UTIs |
| – PCP (Pneumocystis pneumonia, HIV+) | – Bacterial prostatitis | |
| – Typhoid, Shigellosis, Nocardiosis | ||
| Side Effects | – Rash, SJS/TEN (severe skin reactions) | – Hyperkalemia |
| – Bone marrow suppression (↓WBC, platelets) | – Megaloblastic anemia (high dose) | |
| – Crystalluria, Photosensitivity | ||
| Drug Interactions | – ↑ Warfarin effect (bleeding risk) | – ↑ Risk of hyperkalemia with ACEIs/ARBs |
| – ↑ Phenytoin toxicity | ||
| – Avoid with methotrexate (folate antagonism) |
Important Cautions/ Contraindications
-
Avoid in pregnancy (esp. 1st trimester & near term)
-
Contraindicated in G6PD deficiency → may cause hemolysis
-
Use with caution in renal impairment
MCQs: Cotrimoxazole & Trimethoprim
1. Cotrimoxazole is a combination of which two drugs?
A. Ampicillin and Clavulanic acid
B. Sulfamethoxazole and Trimethoprim
C. Penicillin and Sulbactam
D. Amoxicillin and Ciprofloxacin
Answer: B. Sulfamethoxazole and Trimethoprim
2. What is the mechanism of action of Trimethoprim?
A. Inhibits DNA gyrase
B. Inhibits cell wall synthesis
C. Inhibits dihydrofolate reductase
D. Inhibits RNA polymerase
Answer: C. Inhibits dihydrofolate reductase
3. Which of the following is a common use of Cotrimoxazole?
A. Fungal infection
B. Malaria
C. Pneumocystis jirovecii pneumonia
D. Viral hepatitis
Answer: C. Pneumocystis jirovecii pneumonia
4. Which serious dermatologic side effect is associated with Cotrimoxazole?
A. Acne
B. Eczema
C. Stevens-Johnson syndrome
D. Rosacea
Answer: C. Stevens-Johnson syndrome
5. Cotrimoxazole should be avoided in which condition?
A. Asthma
B. G6PD deficiency
C. Diabetes
D. Hypothyroidism
Answer: B. G6PD deficiency
6. Which electrolyte abnormality is commonly seen with Trimethoprim?
A. Hypokalemia
B. Hypernatremia
C. Hyperkalemia
D. Hypocalcemia
Answer: C. Hyperkalemia