Adverse antibiotic drug interactions.

There is enormous potential for drug interactions in patients who, today, often receive many drugs. Antibiotics are prominent amongst the groups of drugs commonly prescribed. Many interactions take place at the absorption stage. Antacids and antidiarrhoeal preparations, in particular, can delay and reduce the absorption of antibiotics such as tetracyclines and clindamycin, by combining with them in the gastrointestinal tract to form chelates or complexes. Other drugs can affect gastric motility, which in turn often controls the rate at which antibiotics are absorbed. Some broad spectrum antibiotics can alter the bacterial flora of the gut which may be related to malabsorption states. The potentiation of toxic side effects of one drug by another is a common type of interaction. Antibiotics which are implicated in this type of interaction are those which themselves possess some toxicity such as aminoglycosides, some cephalosporins, tetracyclines and colistin. Some of the most important adverse interactions with antibiotics are those which involve other drugs which have a low toxicity/efficacy ratio. These include anticoagulants such as warfarin, anticonvulsants such as phenytoin and phenobarbitone and oral antidiabetic drugs like tolbutamide. Risk of interaction arises when the metabolism of these drugs is inhibited by liver microsomal enzyme inhibitors such as some sulphonamides and chloramphenicol, or is enhanced by enzyme inducers such as rifampicin.