Penicillin
Some people are allergic to penicillin. Side effects of taking antibiotics may include nausea, diarrhea, or a rash. Rarely, patients who are allergic to penicillin get a fever, vomit, or have serious skin irritation.  Because it is such a popular antibiotic, penicillin is the most common cause of serious allergic reactions to a drug. They are now used regularly in hospitals but because it is so common in modern medicine, strains of bacteria have developed resistance against the antibiotic (the antibiotic can no longer kill the bacteria).