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The importance of sulphonamides has decreased as a result of both increasing resistance and the emergence of newer, less toxic alternatives.
Sulphonamides act by blocking an early stage in folate synthesis - the condensation of PABA and dihydropteridine to dihydropteric acid.
They are bacteriostatic drugs since it may take some generations for the folate pool in the bacteria to decrease.
Sulphonamides and trimethoprim have been used in combination because of their synergistic activity - as co-trimoxazole.