Tautomerism in the field of metabolism has likewise received little attention, though most drugs are metabolized in the body. The most commonly represented tautomer is considered for analyzing the metabolism of drug molecules. Different tautomers can be metabolized by enzymes or converted non-enzymatically at different rates. Warfarin, an anti-coagulant agent, exists in two enantiomeric forms and has at least 40 tautomers ranging from open chain to ring tautomers. Studies on warfarin and analogue compounds have suggested that tautomeric S-warfarin forms are metabolized in ring form. The pharmacodynamics effect will differ if minor or major tautomers are metabolized rapidly. After one or two rounds of initial oxidation, drugs may go for tautomerization due to the addition of a hydroxyl group or epoxide in the structure. For example, cyclophosphamide is converted to 4-OH-cyclophosphamide via hydroxylation, then this ring tautomer is converted to a chain tautomer (Figure ). Another interesting example is daclatasvir, which is a selective inhibitor of the hepatitis C virus. After δ-oxidation at the pyrrolidine moiety, daclatasvir is prone to tautomeric equilibrium with a chain tautomer (aminoaldehyde) via opening of the pyrrolidine ring. This is converted to a stable metabolite by attacking the aldehyde group with imidazole nitrogen. Mechanism-based inhibition (MBI) is a special type of inhibition where a metabolite formed from a drug is quite reactive and can bind to a metabolizing enzyme covalently or non-covalently, leading to temporary or permanent loss of metabolizing enzyme activity, hence affecting the metabolism of other drugs (drug-drug interaction). These end metabolites can have the potential for tautomerization, and only one of the tautomers leads to MBI (lapatinib nitroso tautomer, (Figure )).