Lidocaine and phenytoin- both are sodium channel blockers. Lidocaine is a local anaesthetic and an anti-arrhythmic. Phenytoin is an anticonvulsant. What explains their differential action? Is it because of their difference in pharmacokinetics.

 Drugs, more than often, have multiple actions. The ability of drugs to interact with different targets and sometimes same target in different tissues impart this promiscuous property to drugs. For example Na+ channels are ubiquitous in the body. There are two types of Na+ channels- voltage gated  and epithelial. Voltage gated Na+ channels are present mainly in the brain, heart and muscles. Epithelial Na+ channels are present in kidney and skin. Drugs which interact with the Na+ channels, therefore, have widespread actions in the body. Phenytoin which is used as an anticonvulsant acts by inhibiting the Na+ channels in the brain, but is also arrhythmogenic due to its action on the cardiac Na+ channels. Similarly, lignocaine which is a local anaesthetic (blocks epithelial Na+ channel) when infiltrated subcutaneously, and is also used as an anti-arrhythmic drug (blocks cardiac Na+ channels). However, the rapid elimination of lignocaine does not allow the attainment of a sufficient level in the brain to act as an anticonvulsant. We understand that the pharmacodynamics is important for the action of a drug, but a drug that does not reaches the target fail to exert any influence, making pharmacokinetics equally important for drug action. 

Soldiers fight a battle to win a war (pharmacodynamics) and the warships that take the soldiers to the battlefield (pharmacokinetics) are the war-makers.