People in pain may well be depressed by their struggle and need treatment for their depression. However, the medicine used against depression have an action against pain which is completely separate from their action in depression. These medicines act by increasing the level in the brain of neurotransmitters, which are used to carry nerve impulses from one cell to another. One of these chemicals, called serotonin or 5-hydroxy-tryptamine, improves the mood. However, the same chemical is the transmitter used by one of the systems by which the brain controls the nerve impulses that arise in the spinal cord and signal injury to the brain. In this way, antidepressants can decrease the incoming signal from cord to brain and improve pain relief. They are used where the narcotics do not work because nerve damage has inactivated the inhibitory mechanisms that are normally activated by narcotics. They are used to treat the pains arising after shingles, which I mentioned earlier. But the effect is weak and these drugs only just pass rigorous tests as analgesics.

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