Trimethoprim

Question 1

What is co-trimazole?

Answer 1

Trimethoprim and sulfamethoxazol

Question 2

What is the method of action of trimethoprim?

Answer 2

• Inhibits bacterial folate synthesis
• Slows bacterial growth (bacteriostatic)
• Bacteria are unable to use external sources of folate so need to make their own for processes like DNA synthesis
• Broad spectrum for gram negative and gram positive bacteria esp. enterobacteria (E.coli)
• Sulfamethoxazole also inhibits folate synthesis

Question 3

What are the indications for trimethoprim?

Answer 3

• Uncomplicated UTI

* Treatment and prevention of pneumocystic pneumonia in immunosuppressed people (Co-trimazole)

Question 4

What are the contraindications for trimethoprim?

Answer 4

• 1st trimester of pregnancy – due to being a folate antagonist so can cause cardiovascular defects and cleft lip in the foetus
• Folate deficiency – caution as ↑ haematological affects
• Renal impairment – dose ↓ due to rise in serum creatinine levels
• Neonates – susceptible to adverse effects
• Elderly – susceptible to adverse effects
• HIV infection – susceptible to adverse effects

Question 5

What are the side effects of trimethoprim?

Answer 5

• GI upset – nausea, vomiting, sore mouth
• Skin rash and hypersensitivity – more common with sulphonamides
• Haematological disorders e.g. megoblastic anaemia, leucopenia, thrombocytopenia – due to being a folate antagonist
• Hyperkalaemia

Question 6

What drugs interact with trimethoprim?

Answer 6

• Aldosterone antagonists – K+ elevating drugs
• ACE inhibitors – K+ elevating
• Angiotensin receptor blockers – K+ elevating
• Methotrexate – folate antagonist
• Phenytoin - ↑ folate metabolism so haematological effects more likely
• Warfarin - ↑ anticoagulant effect by killing normal gut flora that synthesises Vit K