Donating blood is a great way to help others, and it’s easy to do if you’re relatively healthy.

Of course, some people aren’t able to donate blood due to medical conditions or because they’re taking prescription medications. But even a basic, over-the-counter medicine can stop you from donating parts of your blood -- like, you can’t donate platelets if you’ve recently taken aspirin.

Because even though you might’ve just taken it for a headache, aspirin also affects the rest of your body, including your platelets. When you get a cut, it’s your platelets -- also called thrombocytes -- that are mostly responsible for stopping any active bleeding. Along with other proteins, they clump and bind together, making a scab for your open wound.

When someone has a low platelet count, that puts them at risk of severe bleeding, even for a less serious injury. That’s where donations come in. Unfortunately, you’ll probably be turned away from donating if you’ve taken aspirin too recently, because it stops your platelets from working normally.

Aspirin is a complicated molecule made of carbon, oxygen, and hydrogen. It’s used to relieve pain, and for patients at risk for recurring strokes or heart attacks. People have been using it 1890s, but it wasn’t until the 1970s that scientists actually understood how it works.

The drug blocks an enzyme called COX-1. That reduces the production of prostaglandins, hormone-like compounds that normally cause inflammation and pain. Blocking those compounds is what helps your headache.

But the aspirin also inhibits the protein thromboxane. Thromboxane helps create blood clots and scabs by constricting your blood vessels and recruiting platelets to clump together. So, if you take aspirin, which stops thromboxane, that prevents blockages from forming in your blood vessels and keeps your blood flowing free.

That’s why it’s so helpful in preventing heart attacks, which are caused by blood clots, and so harmful for hospital patients. Donor platelets are often used for cancer patients or for complications in surgery, so hospitals need to be completely sure that they’ll actually work and will prevent excessive bleeding. And if your platelets aren’t clumping together like normal, you can see why donating them to someone in need might be a problem.

If you’re interested in donating platelets, most donor banks suggest waiting about 48 hours after taking aspirin to let the drug’s effects wear off so that your platelets can gain back their regular, clotting selves. But if you’re scheduled to donate whole blood—everything, not just the platelets—and you have a headache, no worries! Whole blood transfusions are often used in more general cases where the clotting property of blood isn’t necessarily the main thing they need.

So if your platelets don’t work normally, it’s not as much of a problem. For such a simple and easily available medicine, aspirin does a lot more to your body than you might think. But as long as you plan ahead, you can still go forth, donate, and help those in need.

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