Aspirin contains one active ingredient: acetylsalicylic acid. Every aspirin tablet, regardless of brand, is built around this single compound. The rest of the tablet is a mix of inactive ingredients that hold it together, help it dissolve, and protect your stomach. A standard tablet contains either 325 mg of acetylsalicylic acid (regular strength) or 81 mg (low-dose), with everything else in the pill serving a supporting role.
The Active Ingredient
Acetylsalicylic acid is a synthetic compound with roots in nature. For more than 3,500 years, people chewed willow bark to relieve pain and fever. The bark contains a natural chemical called salicin, which the body converts into salicylic acid. The problem with pure salicylic acid is that it’s harsh on the stomach. Modern aspirin solves this by chemically modifying salicylic acid: in manufacturing, salicylic acid is combined with acetic anhydride (a derivative of vinegar) using a small amount of sulfuric acid as a catalyst. The result is acetylsalicylic acid, which is gentler to swallow but still delivers the same pain-relieving effects once it breaks down in your body.
Inactive Ingredients in the Tablet
The acetylsalicylic acid alone would be a loose powder. To turn it into a pill you can swallow, manufacturers add several types of inactive ingredients, each with a specific job:
- Fillers (like calcium phosphate or lactose) bulk up the tablet to a size that’s easy to handle.
- Binders (like corn starch or mannitol) hold the powder together so the tablet doesn’t crumble.
- Disintegrants (like sodium starch glycolate or crosslinked cellulose) do the opposite of binders. They help the tablet break apart once it reaches your stomach so the active ingredient can dissolve.
- Lubricants (like magnesium stearate) prevent the mixture from sticking to the manufacturing equipment during pressing.
- Glidants (like talc or colloidal silicon dioxide) improve powder flow during production, ensuring each tablet gets a consistent dose.
Some tablets also include colorants, coating agents like shellac or carnauba wax, and sweeteners if they’re chewable. The exact combination varies by brand, but the categories are the same.
What’s Different in Buffered Aspirin
Buffered aspirin adds antacid compounds to the standard formula. A typical buffered tablet contains 325 mg of aspirin along with calcium carbonate, magnesium carbonate, and magnesium oxide. These buffering agents neutralize some of the acid in your stomach, which can reduce the irritation that plain aspirin sometimes causes. If you’ve ever noticed that certain aspirin brands advertise being “gentle” or “easy on the stomach,” the buffering agents are usually why.
What’s Different in Enteric-Coated Aspirin
Enteric-coated aspirin takes a different approach to stomach protection. Instead of adding antacids, the tablet is wrapped in a special polymer coating that resists dissolving in the acidic environment of your stomach. The coating only breaks down once the tablet reaches the more alkaline environment of your small intestine. This means the aspirin bypasses your stomach entirely, which reduces the chance of irritation or ulcers. Enteric-coated versions are especially common in low-dose (81 mg) tablets designed for daily cardiovascular use.
How Aspirin Works in Your Body
Once the tablet dissolves, acetylsalicylic acid is absorbed primarily through the stomach lining and small intestine. Inside your body, it targets enzymes called COX-1 and COX-2. These enzymes produce chemicals called prostaglandins, which trigger pain, inflammation, and fever. Aspirin permanently disables these enzymes by attaching an acetyl group to them, which is why its effects last longer than you might expect from a drug that clears your bloodstream relatively quickly.
This same mechanism affects platelets, the tiny blood cells responsible for clotting. Platelets rely on COX-1 to clump together, and since aspirin’s block is permanent and platelets can’t repair themselves, a single dose keeps your blood slightly thinner for the entire lifespan of those platelets (about 7 to 10 days). This is why low-dose aspirin is used for heart protection, and also why surgeons ask you to stop taking it before procedures.
Low-Dose vs. Regular Strength
The amount of acetylsalicylic acid in the tablet determines what aspirin does best. Low-dose aspirin, typically 81 mg, is the standard for cardiovascular prevention. At this dose, aspirin primarily blocks platelet clumping without suppressing as much of the body’s overall prostaglandin production. Regular-strength aspirin at 325 mg (and higher doses up to 650 mg for two tablets) delivers stronger pain relief, fever reduction, and anti-inflammatory effects.
Current guidelines from the U.S. Preventive Services Task Force are cautious about daily aspirin for heart protection. For adults 40 to 59 with elevated cardiovascular risk, the decision is considered individual, with the net benefit described as small. For adults 60 and older, the task force recommends against starting daily aspirin for prevention because the bleeding risks tend to outweigh the benefits. These recommendations apply to people without existing heart disease. If you’ve already had a heart attack or stroke, aspirin use is a separate conversation with different considerations.
One Important Safety Note for Children
Aspirin should not be given to children or teenagers, particularly during viral illnesses like the flu or chickenpox. It has been linked to Reye’s syndrome, a rare but serious condition that causes swelling in the liver and brain. The one exception is children with certain chronic conditions like Kawasaki disease, where aspirin may be part of a treatment plan under medical supervision.