How do drugs know where to go in the body? A pharmaceutical scientist explains why some medications are swallowed while others are injected

While pills come in many shapes and sizes, they all eventually reach your bloodstream and travel throughout your body. Vadim Sazhniev/iStock via Getty Images

When you take aspirin for a headache, how does the aspirin know to travel to your head and alleviate the pain?

The short answer is, it doesn’t: Molecules can’t transport themselves through the body, and they don’t have control over where they eventually end up. But researchers can chemically modify drug molecules to make sure that they bind strongly to the places we want them and weakly to the places we don’t.