You swallow one tablet for a throbbing headache, and twenty minutes later your head feels better. Your knee, your back and your stomach feel exactly the same. So how did the pill know where to go? This question is the subject of a popular explainer video from The Brain Maze (watch it here), and the answer surprises most people. We dug into the research to explain it in full.
Quick answer: A painkiller does not know where it hurts. Once it enters your blood, it travels to almost every part of your body. It only seems targeted because the injured area is the one place producing the pain chemicals (or sending the pain signals) that the drug blocks. Where nothing is wrong, there is nothing for the drug to switch off, so you notice no effect there.
The big misconception: pills don’t travel to the pain
When you swallow a tablet, it dissolves in your stomach, is absorbed mostly through the small intestine, passes through the liver, and then enters your bloodstream. From there, your heart pumps it around your entire body. Your brain, toes, liver, kidneys and the sore spot all receive roughly the same drug.
There is no GPS, no homing signal and no “pain address” written on the pill. The trick is not in the drug. It is in the chemistry of the injury itself.
What actually happens when you get hurt
To understand painkillers, you first need to understand pain. Your body is wired with special nerve endings called nociceptors. They are pain sensors, and they are spread through your skin, muscles, joints and organs.
- Damage happens. You stub a toe, strain a muscle or develop an infection.
- Damaged cells release chemicals. One of the most important groups is prostaglandins. They are made by enzymes called cyclooxygenase, or COX for short.
- Nerves become extra sensitive. Prostaglandins lower the firing threshold of the nociceptors around the injury, so even light pressure now feels painful. They also drive swelling, redness and heat.
- The signal travels up. The nociceptor fires an electrical signal, carried by sodium channels in the nerve, through the spinal cord to the brain.
- The brain decides it hurts. Pain is only “felt” once the brain processes that signal.
Every type of painkiller works by interrupting one or more of these steps. And that is the key: most of these steps are only active at the injury.
Ibuprofen and aspirin: shutting down the pain factory
Ibuprofen, aspirin, naproxen and diclofenac belong to a family called NSAIDs (non-steroidal anti-inflammatory drugs). They block the COX enzymes, so the body cannot make new prostaglandins.
Imagine a city where only one factory is running and pumping out smoke. If you cut the power to every factory in the city, you only notice the change at the one that was actually running. That is what NSAIDs do. They reach every tissue, but prostaglandin production is ramped up at the injured spot, so that is where the effect shows up. The nerves there calm down, swelling eases, and the pain fades.
Paracetamol: the mystery drug that got a new explanation
Paracetamol (called acetaminophen in the US) has been used for more than a century, yet scientists have argued for decades about exactly how it works. It is a weak anti-inflammatory, so it can’t just be blocking prostaglandins at the injury like ibuprofen. The long-standing view was that it acts mainly in the brain and spinal cord.
A 2025 study published in PNAS by researchers at the Hebrew University of Jerusalem, led by Prof. Alexander Binshtok and Prof. Avi Priel, added an important twist. They found that the body converts paracetamol into a metabolite called AM404, and that this compound is formed inside the pain-sensing nerve endings themselves. There, AM404 blocks the sodium channels Nav1.7 and Nav1.8, which nerves need to fire pain signals. It binds the same site that local anesthetics use.
“This is the first time we’ve shown that AM404 works directly on the nerves outside the brain.” – Prof. Alexander Binshtok
Because these sodium channels are mainly involved in active pain-sensing nerves, this helps explain why paracetamol eases pain without numbing everything else. The researchers say it could also point toward new pain drugs with fewer side effects.
Opioids: changing how the brain hears the signal
Strong prescription painkillers such as morphine, codeine and oxycodone work differently. They attach to mu-opioid receptors in the brain and spinal cord. Instead of stopping the pain signal at its source, they turn down the volume of the signal and change how the brain perceives it. Many people describe still feeling the pain but no longer being bothered by it.
This is the one class where the drug genuinely does work on the whole system, which is also why opioids come with serious risks such as drowsiness, slowed breathing, constipation and dependence. They are prescription-only for good reason.
Local anesthetics: the one type that really is targeted
Drugs like lidocaine, used by dentists or in numbing creams, are the exception. They are put exactly where they are needed. They block the sodium channels in nearby nerves, so no signal can leave that area at all. Here the drug “knows” where the pain is only because a person placed it there.
Painkillers compared at a glance
| Type | Examples | Where it acts | How it seems to “find” the pain |
|---|---|---|---|
| NSAIDs | Ibuprofen, aspirin, naproxen | Whole body, effect at the injury | Blocks COX enzymes; prostaglandins are mostly being made at the injured spot |
| Paracetamol | Paracetamol / acetaminophen | Brain, spinal cord and pain-sensing nerve endings | Metabolite AM404 blocks Nav1.7/Nav1.8 in active pain nerves (2025 research) |
| Opioids | Morphine, codeine, oxycodone | Brain and spinal cord | Turns down pain signals and changes perception of pain |
| Local anesthetics | Lidocaine, benzocaine | Only where applied | Physically placed at the site; blocks all nerve signals there |
Why side effects prove the drug is everywhere
If painkillers really went only to the sore spot, they would have almost no side effects. But they do, and the side effects show up in places that have nothing to do with your pain:
- Stomach problems with NSAIDs. The COX-1 enzyme also makes prostaglandins that protect your stomach lining. Block it everywhere and that protection weakens, which is why ibuprofen can cause irritation or ulcers, especially on an empty stomach or with long-term use.
- Liver risk with paracetamol. The liver processes paracetamol. Taken above the recommended amount, a toxic by-product builds up and can seriously damage the liver.
- Drowsiness and constipation with opioids. Opioid receptors exist in the gut and many brain areas, not just pain pathways.
These effects are the clearest proof that the drug is circulating through your entire body.
Why does it take 20 to 30 minutes to work?
The delay is the time it takes for the tablet to dissolve, be absorbed, pass the liver and reach a high enough level in the blood. Food in your stomach can slow this down. Gels, liquids and soluble tablets are sometimes absorbed a little faster, but they still have to make the same full-body journey.
Frequently asked questions
Does a painkiller go straight to the pain?
No. Swallowed painkillers spread through the bloodstream to the whole body. They only have a noticeable effect where pain chemicals or pain signals are active.
Why does ibuprofen reduce swelling but paracetamol doesn’t much?
Ibuprofen blocks prostaglandin production at the injury, which drives both pain and inflammation. Paracetamol has very little anti-inflammatory effect in the body’s tissues and works more on pain signalling in nerves and the central nervous system.
Can I take ibuprofen and paracetamol together?
They work in different ways, and doctors sometimes recommend combining or alternating them. But whether it is right for you depends on your health, age and other medicines. Ask a pharmacist or doctor and always follow the label.
Why do painkillers work on headaches if nothing is injured?
Headaches still involve active pain pathways, including inflamed blood vessels and sensitised nerves around the head. The drug acts where that activity is happening.
Are painkillers addictive?
Common over-the-counter painkillers like ibuprofen and paracetamol are not addictive in the way opioids are, though overuse can cause “medication overuse headaches” and organ damage. Opioids carry a real risk of dependence.
The bottom line
A painkiller is less like a guided missile and more like a city-wide power cut that you only notice at the one building with its lights on. The drug spreads everywhere. Your injury is simply the place where the pain chemistry is running, so that is where the relief appears. And as new research on paracetamol shows, scientists are still uncovering exactly how some of our most familiar medicines work.
Not medical advice: This article is for general education only. Always read the label, stay within the recommended dose, and talk to a doctor or pharmacist before combining medicines, or if you have liver, kidney, stomach or heart conditions, are pregnant, or are giving painkillers to a child.
Sources: The Brain Maze, “How Does a Painkiller Know Where It Hurts?” (YouTube); Hebrew University of Jerusalem study on AM404, PNAS (2025), as reported by Neuroscience News; How It Works magazine; NCBI StatPearls entries on NSAIDs, acetaminophen and opioids.
