Advil (ibuprofen) works by blocking enzymes in your body that produce chemicals called prostaglandins, which trigger pain, inflammation, and fever. When you take a dose, the drug binds to these enzymes and temporarily shuts down their activity, reducing all three symptoms at once. Most people notice relief within about 20 minutes of swallowing a 400 mg tablet.
The Enzyme It Targets
Your body contains two closely related enzymes called COX-1 and COX-2 (short for cyclooxygenase). These enzymes are the starting point for producing prostaglandins, a family of chemical messengers involved in pain signaling, inflammation, and temperature regulation. Ibuprofen binds to both COX-1 and COX-2, physically blocking the spot where the raw material (a fatty acid called arachidonic acid) would normally attach. This is a competitive process: the drug and the fatty acid are essentially fighting for the same seat.
Unlike aspirin, which permanently disables COX enzymes, ibuprofen’s grip is temporary and reversible. It latches on, blocks prostaglandin production for a few hours, then lets go. That’s why the effects wear off and you need another dose.
What Prostaglandins Actually Do
Prostaglandins are not inherently harmful. They play roles throughout the body, from protecting your stomach lining to regulating blood flow in your kidneys. But when tissue is injured or infected, your cells ramp up prostaglandin production dramatically, and that surge creates the familiar signs of inflammation: redness, swelling, and pain.
One prostaglandin in particular, PGE2, is involved in all three of those responses. It widens blood vessels near the injury site, increasing blood flow (redness) and allowing fluid to leak into surrounding tissue (swelling). It also sensitizes nerve endings at the injury and in the spinal cord, making them fire pain signals more easily. This heightened sensitivity, called hyperalgesia, is why a swollen ankle hurts when you barely touch it. By cutting prostaglandin production, ibuprofen dials down all of these effects simultaneously.
Prostaglandins also act on the brain’s temperature-control center to raise your body temperature during infection. Blocking their production is how ibuprofen reduces fever.
How Quickly It Works and How Long It Lasts
After swallowing a standard 400 mg tablet, most people experience noticeable pain relief within 20 minutes. In clinical trials measuring plasma levels, the average blood concentration at the 20-minute mark was already 8.4 micrograms per milliliter, enough to begin meaningful COX inhibition. A 200 mg dose also showed significant relief at 20 minutes in at least one trial, though peak effect takes longer.
The effects of a single dose typically last four to six hours. The drug is almost entirely processed by your liver, which converts it into inactive byproducts. These metabolites are then filtered out through your kidneys. Very little unchanged ibuprofen ends up in your urine. The two main breakdown products account for roughly 60% of the dose you took.
Why It Can Upset Your Stomach
The same prostaglandins that cause pain and swelling at an injury site also do something useful in your stomach: they maintain the protective mucus layer that keeps digestive acid from eating into your stomach wall. Because ibuprofen blocks COX-1 throughout the body (not just at the injury), it reduces prostaglandin levels in the stomach too. This weakens that protective barrier.
For occasional use, this is rarely a problem. But with regular daily use, the loss of that mucosal protection becomes a real concern. Over weeks and months, it can lead to stomach ulcers. Taking ibuprofen with food or a glass of milk helps buffer the immediate irritation, but it doesn’t fully prevent the underlying mechanism.
How It Differs From Acetaminophen
Acetaminophen (Tylenol) reduces pain and fever but has almost no effect on inflammation. It works primarily in the brain rather than at the site of injury, which is why it doesn’t cause the same stomach problems. For conditions where inflammation is the main driver of pain, like a sprained ankle or arthritis flare, ibuprofen has a theoretical advantage because it targets the inflammation itself.
In practice, however, the difference isn’t always dramatic. A clinical trial comparing acetaminophen to both low-dose and high-dose ibuprofen in people with knee osteoarthritis found similar improvements in pain scores across all three groups over four weeks. Side effects were also comparable. For straightforward pain relief without significant swelling, either drug can work well.
Cardiovascular and Drug Interaction Risks
All non-aspirin NSAIDs, including ibuprofen, carry a slightly increased risk of heart attack and stroke. This applies to people with and without existing heart disease, though the risk is higher for those who already have cardiovascular problems. The risk can increase with longer use, and serious events have occurred as early as the first few weeks of daily use. The safest approach is to use the lowest effective dose for the shortest time you need it.
One interaction worth knowing about: ibuprofen can interfere with low-dose aspirin’s ability to prevent blood clots. Both drugs compete for the same binding site on the COX enzyme in platelets, but aspirin works by permanently disabling the enzyme while ibuprofen only blocks it temporarily. If ibuprofen gets there first, it can prevent aspirin from doing its job. The FDA notes this interference occurs when ibuprofen is taken within 8 hours before or 30 minutes after an aspirin dose. If you take daily low-dose aspirin for heart protection, the timing of ibuprofen matters. Acetaminophen does not cause this interaction.
Use in Children
Ibuprofen can be given to children 6 months and older for pain and fever, dosed by weight rather than age for accuracy. It can be repeated every 6 to 8 hours as needed. The FDA has not approved its use in infants under 6 months because safety data in that age group is insufficient.