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Metronidazole is active against most obligate anaerobic bacteria and protozoa by undergoing intracellular chemical reduction via mechanisms unique to anaerobic metabolism within the organism. Reduced metronidazole, which is cytotoxic but short-lived, interacts with microorganism's DNA to cause a loss of helical structure, strand breakage and resultant inhibition of nucleic acid synthesis and cell death.
Metronidazole is well absorbed orally with a bioavailability of at least 80%. Peak plasma concentrations of 6, 12 and 40 mcg/mL are achieved within 1 to 2 hours of a 250 mg, 500 mg and 2 g single oral doses respectively. Absorption may be delayed, but is not reduced overall by administration with food.
Metronidazole is widely distributed. It appears in most body tissues and fluids including bile, bone, breast milk, saliva, seminal fluid, liver and liver abscesses, lungs and vaginal secretions. It also crosses the placenta and the blood brain barrier. Not more than 20% is bound to plasma proteins.
Metronidazole is metabolised in the liver by side-chain oxidation and glucuronide conjugation to 2-hydroxymethyl (also active) and other metabolites. The half-life of metronidazole in normal adults is 8 hours and in patients with alcoholic liver disease 18 hours.
Metronidazole and its primary metabolites are rapidly removed from the blood by haemodialysis but is not significantly removed by peritoneal dialysis.
