Which of the following is a common side effect of penicillin?

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Penicillin often leads to gastrointestinal upset because antibiotics disrupt the normal gut bacteria. This disturbance can cause diarrhea as the balance of microbes in the intestines is altered, sometimes allowing overgrowth of other organisms or irritating the gut. Broad-spectrum penicillins, in particular, tend to have a higher risk of this effect. The other options—hypertension, hair loss, and dizziness—are not typical or characteristic side effects of penicillin, so diarrhea stands out as the common reaction associated with this antibiotic.