Which enzyme does sulfamethoxazole competitively inhibit?

Sulfamethoxazole, as a sulfonamide, acts at an early step in folic acid synthesis. Its target is dihydropteroate synthase, acting as a structural analog of para-aminobenzoic acid (PABA) to competitively block the formation of dihydropteroate.

Which microorganisms are included in the antibacterial spectrum of co-trimoxazole?

Co-trimoxazole has a broad spectrum of activity against many gram-positive and gram-negative aerobic bacteria. Highly susceptible organisms include Haemophilus influenzae, Neisseria meningitidis, Streptococcus pneumoniae, and certain Enterobacteriaceae (such as E. coli).

Why is co-trimoxazole more effective than individual sulfonamides?

The combination blocks two distinct steps in the bacterial folate synthesis pathway. For a bacterium to survive, simultaneous mutations in two separate genetic loci would be required, which is extremely unlikely.

What type of anemia can develop with prolonged antifolate drug therapy?

Administration of folic acid antagonists (including trimethoprim) can lead to macrocytic megaloblastic anemia due to impaired DNA synthesis in erythroid precursors.

Can folic acid be administered alone in pernicious anemia?

No, this is contraindicated. Folic acid in pernicious anemia must always be administered alongside vitamin B12; otherwise, subacute combined degeneration of the spinal cord and other neurological deficits may progress.

Home › Pharmacology › Co-Trimoxazole: Mechanism, Uses and Side Effects

Co-Trimoxazole

For medical students2 min readUpdated 2026-10-10

Co-Trimoxazole is a fixed-dose combination antibacterial agent consisting of sulfamethoxazole and trimethoprim. It acts as a combination of a sulfonamide and a dihydrofolate reductase inhibitor, providing a synergistic bactericidal effect.

CompositionSulfamethoxazole (0.4 g) + Trimethoprim (0.08 g)

MechanismSequential blockade of folate synthesis at two consecutive steps

Type of ActionBactericidal (synergistic component interaction)

AdministrationOral, 2 tablets twice daily after meals

Mechanism of Action

The drug blocks microbial folic acid metabolism. Sulfamethoxazole competitively inhibits dihydropteroate synthase to block dihydrofolic acid synthesis, while trimethoprim inhibits the enzyme dihydrofolate reductase, preventing the conversion of dihydrofolate into its active coenzyme form, tetrahydrofolate. This sequential dual blockade of the metabolic pathway produces synergy and significantly delays the development of bacterial resistance.

Pharmacological Effects

Unlike sulfonamide monotherapy, this combination exerts a bactericidal effect. The drug is well absorbed from the gastrointestinal tract and widely distributed into tissues, achieving high concentrations in bronchial secretions, bile, urine, and prostatic tissue. It crosses the blood-brain barrier, which is clinically important for central nervous system infections.

Indications

Co-trimoxazole is effective for infections caused by Haemophilus influenzae and is the drug of choice for the treatment and prophylaxis of Pneumocystis jirovecii (formerly Pneumocystis carinii) pneumonia. It is also used as an alternative regimen in toxoplasmosis.

Adverse Effects and Contraindications

The primary risk stems from its antifolate activity: prolonged administration can cause macrocytic megaloblastic anemia due to folate deficiency. The drug is contraindicated during the first trimester of pregnancy and near term due to the risk of teratogenicity and kernicterus in newborns.

Clinical Considerations

The components are selected based on matching elimination half-lives (approximately 12 hours). When managing anemias caused by folate deficiency, it is critical to remember that in pernicious anemia, folic acid supplementation must be strictly combined with vitamin B12 to prevent the progression of neurological damage.

Mnemonic

Co-Trimoxazole: "Double punch" — blocking two consecutive enzymes so the bacteria cannot easily mutate to develop resistance.

Frequently asked questions

Which enzyme does sulfamethoxazole competitively inhibit?: Sulfamethoxazole, as a sulfonamide, acts at an early step in folic acid synthesis. Its target is dihydropteroate synthase, acting as a structural analog of para-aminobenzoic acid (PABA) to competitively block the formation of dihydropteroate.
Which microorganisms are included in the antibacterial spectrum of co-trimoxazole?: Co-trimoxazole has a broad spectrum of activity against many gram-positive and gram-negative aerobic bacteria. Highly susceptible organisms include Haemophilus influenzae, Neisseria meningitidis, Streptococcus pneumoniae, and certain Enterobacteriaceae (such as E. coli).
Why is co-trimoxazole more effective than individual sulfonamides?: The combination blocks two distinct steps in the bacterial folate synthesis pathway. For a bacterium to survive, simultaneous mutations in two separate genetic loci would be required, which is extremely unlikely.
What type of anemia can develop with prolonged antifolate drug therapy?: Administration of folic acid antagonists (including trimethoprim) can lead to macrocytic megaloblastic anemia due to impaired DNA synthesis in erythroid precursors.
Can folic acid be administered alone in pernicious anemia?: No, this is contraindicated. Folic acid in pernicious anemia must always be administered alongside vitamin B12; otherwise, subacute combined degeneration of the spinal cord and other neurological deficits may progress.

Go deeper

Comparative pharmacodynamics of short- and long-acting sulfonamides.
Mechanisms of bacterial resistance to antifolate agents.
Pharmacokinetic differences between sulfamethoxazole and sulfadoxine.
The role of folic acid in erythrocyte metabolism and DNA synthesis.
Management of opportunistic infections in immunocompromised patients.