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Local anaesthetics act locally, meaning they don’t affect any area of the body that is not supplied by the blocked nerve or nerves.
This is in contrast with the other painkillers, paracetamol, NSAIDs and the opioids. These painkillers act systemically, i.e. their effect is exerted throughout the body and is not just limited to one area.
When administered, whether orally or by suppository or injection, these drugs have to be absorbed into the bloodstream. Depending on the route of administration and the specific drug, this can take a while – up to half an hour or longer.
Once in the bloodstream the drugs are carried to the organ or body part where they are effective: paracetamol mainly to the brain, where it has an anti-prostaglandin effect; NSAIDs are carried mainly to areas of trauma, inflammation and pain in the body, where they also have an anti-prostaglandin effect; and the opioids to specific receptors in the brain and the spinal cord, where they interfere with pain perception.
The duration of action of local anaesthetics varies from one to almost 20 hours. It depends on:
- the type of local anaesthetic used
- the concentration
- Whether adrenaline has been added to the local anaesthetic or not. If adrenaline has been added, the local blood vessels constrict so that it takes longer for the local anaesthetic to be absorbed into the bloodstream and carried away, resulting in a longer effect.
- the size of the nerve that is blocked
- The area of the body where it is injected: some areas have more blood vessels than others so that the local anaesthetic is carried away by the bloodstream more rapidly, resulting in a shorter duration of effect. For instance, the effect of blocking a nerve between the ribs when the rib has been broken, does not last as long as the effect of blocking the nerves of the wrist.
