For example, take the drug diphenhydramine. The website says that "Diphenhydramine is used to treat sneezing, runny nose, watery eyes, hives, skin rash, itching, and other cold or allergy symptoms." It also says that "Diphenhydramine may cause dizziness or drowsiness."
Question: why is it that diphenhydramine "may" cause dizziness or drowsiness? Are our bodies so different that it can cause dizziness/drowsiness in one person but not in another? And if it's indeed the case that our bodies are different, can it also be the case that diphenhydramine "may" treat sneezing, runny nose etc in some people but not in others?