This is in contrast with the other pain killers, namely paracetamol, NSAIDs and the opioids. These pain killers act systemically in the body, i.e. their effect is exerted all over, not just limited to one area.

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When administered, whether orally, or by suppository or injection, they have to be taken up into the bloodstream. This takes a few minutes, up to half an hour or longer, depending on the route of administration or the specific drug.

Once in the bloodstream, they 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 mainly to areas of trauma, inflammation and pain in the body, where they also have an anti-prostaglandin effect; the opioids to specific receptors in the brain and the spinal cord, interfering with pain perception.

The duration of action of local anaesthetics varies from one hour to almost 20 hours. It depends on:

• the type of local anaesthetic used


• the concentration


• whether adrenaline is added to the local anaesthetic or not: if adrenaline is added, the local blood vessels constrict so that the local anaesthetic is less rapidly taken up by 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 bloodvessels than others so that the local anaesthetic gets carried away by the bloodstream more rapidly, resulting in a shorter duration of effect, e.g. the effect of blocking a nerve between the ribs, like for a broken rib, does not last as long as the effect of blocking the nerves of the wrist.

South African Dental Association (SADA)

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