All local anaesthetics have a lipid soluble ring (Fig 1a) linked to an amine-containing group (Fig 1b). The two groups of local anaesthetic differ in the type of linkage.

The general structure of local anaesthetics and the ester linkage is shown in (Fig 1c). The ester link is more likely to be broken down, as esterases are found in many tissues, including blood.

The following agents are ester local anaesthetics:

• Tetracaine (amethocaine)
• Cocaine

The ester local anaesthetics have a shorter duration of action, but are associated with a higher incidence of hypersensitivity reactions.