Eli5. Adverse reactions

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How does an adverse reaction occur after taking medication?

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Basically, you take a medication and it contains a chemical compound (or group of them) that is intended for a specific purpose. But, the chemicals won’t just go to the place you need them, they go all over the body, and sometimes those chemicals can cause a different part of the body to also react to them and cause problems.

Here is an example:

Aspirin. Aspirin acts as a blood thinner and pain reliever. So when you take it for your heart or a headache, it helps those things. But since it travels all over your body, not just to your heart and head, the blood thinning properties of it could cause you to bleed elsewhere, like in your intestines or something.

Some adverse effects are the nature of the medication. Aspirin, or even more “blood thinners” like warfarin (Coumadin) apixaban (Eliquis), and others can cause bleeding because of how they work. Blood pressure medications can cause too low blood pressure with lightheadedness if the dose is too high. Other times it’s less directly linked to the intended purpose but makes sense because of what the medication does. Antidepressants like sertraline (Zoloft) can cause diarrhea because gut is also full of serotonin receptors even though they have nothing to do with brain. All those are adverse but not really unpredictable.

Other adverse effects are like a severe allergic reaction to penicillin. It has nothing to do with how the medication works, it just happens. Other medications seem to have consistent but not well explained adverse effects, so they probably interact with some cells or proteins in ways that aren’t understood.