Well our boady treat medicines as foreign substance . Because all medicines contain ingredients which our White Blood Cells consider as a threat.
So as soon as this drugs enter our system the boady tries to expel them
Also certain substance in these drugs may change unwanted settings in our boady. That is why some antibiotics give heart attacks and an antidepressant gives us diarrhea
So in conclusion we can say that our boady treats drugs as foreign substances. So while trying to resist this foreign substance a reaction is triggered which is the reason foe side effects in our boady
Hope it helps
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