How come painkillers stop existing pain but not new pain?


Like, if I had a headache I could take painkillers and it would go away, but then I stub my toe and it still hurts the same? Why?

In: Biology

There are many different types of painkiller medications (formally called analgesics), but few pain medicines are able to block out all pain completely.

Relatively mild over-the-counter analgesics like acetaminophen (Tylenol) or ibuprofen (Motrin or Advil) mainly reduce pain by decreasing inflammation. This is particularly helpful for pain that is constant or related to ongoing musculoskeletal inflammation—like that occurring in knee arthritis, tension headaches, and ankle sprains. However, unless the pain is truly minimal to begin with, acute (new) pain is rarely completely blocked by these painkillers.

So when you stub your toe, your nervous system essentially gets the same input that it would without the analgesic medication in your system, and you experience severe pain. (Though if you’d taken enough ibuprofen beforehand, you might have a bit less persistent pain—particularly after the initial throbbing starts to die down.)

That said, some analgesics—especially strong opioids like morphine and fentanyl—can absolutely block acute pain almost completely, especially at higher doses such as those used during anesthesia. (One of the main reasons to undergo anesthesia is that you essentially get enough painkilling effect that intense pain from surgical manipulation is tolerable.) Thing is, doctors rarely prescribe such high doses to people outside the hospital because they are likely to make you extremely sleepy or even stop your breathing—neither of which are great outcomes.

So it’s likely that stubbing your toe will always hurt. (Though shoes might help prevent this.)