How do pain medications target pain?

06 March 2011


Dear Naked Scientists,

I was wondering how Pain relief drugs target pain, and why we don't just go numb in random parts of the body? I hope you can help me in this,

Andrew McCluskey


We put this to Tim Warner from Barts and the London School of Medicine and Dentistry...

Tim - The explanation for this lies in what causes the pain and how we experience it. If we imagine, for instance, pain coming from something like a damaged tissue - you could think of something like an arthritic knee for instance. In that knee, there's a local generation of factors that sensitise the local pain nerve endings. So that local sensitisation depends upon what's happening locally in the knee and then to the nervous pathways, this is then taken as a signal to the brain where we perceive the pain. And so, that part of our perception depends on what's happening in the central nervous system. So we have that as an idea in our heads. We can think about how the pain-relieving drugs work. The non-steroidal drugs that we use for muscle, joint aches and pains, and other drugs like ibuprofen, they stop the formation of the sensitising factors - in this example, the knee, and only the arthritic knee is making the factors. It's only there that the ibuprofen-type drugs act, and so, we feel less pain in our inflamed knee. And at the same time, the other knee doesn't go numb because it isn't making the sensitising factors. So there's nothing there for ibuprofen to inhibit. If you had a more intense pain, say an operation on your knee, so you might use something stronger such as a morphine type drug, and those ones do work within the central nervous system. So those drugs are going to cut down the signals in the brain, coming from the nerve collections in the knee, and so, they cut down the sensation of pain by a central effect in the brain, and not by acting locally in the knee like ibuprofen. But because they act centrally, they have a tendency to also generally dampen down pain pathways. So to some extent, you may have a feeling of numbness somewhere else.

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