An agent added to local anesthetics to speed the onset of action is:

* LAs are weak bases. These require penetration inside the neuron for their action. For entry in the neuron, LAs have to cross the neuronal membrane. * Unionized drugs (lipid soluble) can easily cross the membrane, therefore addition of NaHCO3 in the local anaesthetic solution (weak bases are un-ionized in the alkaline medium) makes them rapid acting. * Adrenaline increases the duration of action by causing vasoconstriction. * Methylparapben is the preservative added in LA solution.