cotrimoxazoleCoTrimoxazole is a rational combination of two antifolate drugs,
  1. Pyrimidine Analogue : Trimethoprim.
  2. Sulfonamide: Sulfamethoxazole.
used in treatment of various diseases where alone drug fails.
  • CoTrimoxazole name itself indicates “ COmbination of TRIMethoprim and SulfamethOXAZOLE”.trimethoprimsulfamethoxazole
  • This combinations offers “Synergistic Effect” hence the benefit is more than used alone.

Mechanism of Action:

  • These drugs attack the bacterial folic acid synthesis pathway at different steps and prevent bacterial folic acid synthesis leading to a “Bactericidal Effect”.
  • Sulfamethoxazole inhibits bacterial enzyme “Folate Synthtase”  and prevents formation of “Dihydrofolic acid from PABA (Para Amino Benzoic Acid).
  • Trimethoprim inhibits bacterial enzyme “Dihydrofolate Reductase” and prevents conversion of “Dihydrofolic acid to Tetrahydrofolic acid.”.

Dose Combination:

  • This is a fixed dose combination that comes in 1:5 ratio of Trimethoprim and Sulfamethoxazole i.e. 80 mg of Trimethoprim is combined with 400 mg of Sulfamethoxazole.
  • Double dose DS combination combines 160 mg of Trimethoprim and 800 mg of Sulfamethoxazole.
  • Lower dose or half dose combines 40 mg of Trimethoprim and 200 mg of Sulfamethoxazole.

Benefits of CoTrimoxazole:

  • Produces synergistic effect “Two drugs acting at different sites to produce similar response.”
  • Spectrum of antimicrobial activity is greater than that of sulfonamide alone
  • Development of resistance is slow.

Why CoTrimoxazole is a rational combination?

  • Both drugs have similar activity i.e Antibacterial.
  • Half lives of both drugs are almost similar i.e. Sulfamethoxazole 10 Hrs and Trimethoprim 8 Hrs.
  • They have different mechanism of action but similar pharmacokinetic profiles.
  • Pharmacokinetics of CoTrimoxazole
    Component Tmax (h) Vd (L) Protein binding t1/2 (h) Excretion
    Sulfamethoxazole 1-4 20 66% 8-10 Renal
    Trimethoprim 1-4 130 42-45% 10 Renal


  1. Urinary tract infection
  2. Respiratory tract infection
  3. Bacterial dysentery
  4. Skin infections & burns.
  5. Malaria
  6. Travelers diarrhea.

Adverse Effects:

  1. Crystaluria – causing kidney damage
  2. Kernicterus(Bilirubin induced brain dysfunction) in foetus & neonates
  3. Hypersensitivity reaction and skin rashes
  4. Aplastic anaemia
  5. Steven Johnson’s syndrome (SJS)


  1. Known hypersensitivity.
  2. Pregnancy.
  3. Neonates bellow 6 weeks.
  4. Hepatic failure.
  5. Renal Failure.

Market Brands:

  1. Bactrim.
  2. Septran.


  • Combination of Trimethoprim and Sulfadiazine is called as “CoTrimazine

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