The selectively toxic effect of nitroimidazole drugs towards anaerobic bacteria and protozoa depends on a number of factors. The killing action of such drugs as metronidazole requires the reduction of the nitro group, a process which influences the rate of entry of the drug into the susceptible cell and which is determined by mechanisms involving ferredoxin-linked (or the equivalent) reactions in the cell. The reduced agent subsequently causes strand breakage of DNA, the extent of which depends on the A + T content of the DNA. Other effects of such drugs may include the possible inhibition of DNA repair mechanisms which exacerbate DNA damage, Inhibition of activity of nitroimidazoles may be caused by aminothiol radical scavengers and radioprotectors normally present in the cell or by the presence of other organisms in the environment (that is, the vagina) capable of inactivating the drugs.