Reversible block by the proper application of local anaesthetic agents is an invaluable clinical aid. These anaesthetic agents belong to the chemical groups of amino-esters, or amino-amides.

Amino-esters
All local anaesthetics in this group in clinical use are esters of para-aminobenzoic acid. The first was procaine, which was the standard drug for more than 40 years, and is still in use. It has no topical anaesthetic properties.

Amino-amides
The first commercially available agent in the series of amino-amides was Lidocaine, which became available in 1944. Because of its significant pharmacological advantages over procaine, lidocaine soon became the standard drug. It is still the most widely used local anaesthetic in dentistry. Lidocaine possesses good topical anaesthetic properties.

Another agent in these series is mepivacaine, which is similar to lidocaine in many respects but lacks topical anaesthetic qualities. The most recent of the agents in this series is prilocaine, a compound similar to lidocaine. Its low systemic toxicity is the main clinical advantage of this potent local anaesthetic agent.

All local anaesthetics in clinical use have a typical chemical arrangement - an aromatic and amine constituent connected by a chain. The aromatic part of the molecule is responsible for the lipophilic properties; the amine end is associated with hydrophilicity. Changes in any of these portions will alter the characteristics of lipid/water solubility and protein-binding. The consequences of such changes will be an altered anaesthetic effect. Such alterations are reflected in changes in intrinsic anaesthetic potency, onset time, duration of action, toxicity ratio, and rate of degration.