The pharmacokinetics study of the tautomer of cefotetan revealed that it is converted to a tautomer with antibacterial activity similar to the parent drug, predominantly in the urine of rabbits and monkeys. However, this tautomer was not observed in the plasma of studied animals including humans (Figure ). Tautomerism may play a role in converting metabolites to final or intermediate metabolites that facilitate their easy excretion. For example, in the case of the drug elzasonan, tautomerism was reported via the formation of the indole iminium ion metabolite through ring closure and rearrangement via proton migration between two carbons, and this metabolite was not detected in urine but excreted through feces. 3. Pharmacodynamics Tautomerism can affect the binding of a drug to its target and the subsequent drug action, including the drug's efficacy and potency. Erythromycin is a frequently prescribed antibiotic that exists in three tautomeric forms (one ketone and two cyclic hemiketals) (Figure ). It has been proven that the ketonic form is active and recognized by ribosomes, inhibiting protein synthesis. The inactive tautomer can be present in up to 20% in the gastrointestinal tract.