Erythromycin
"I was prescribed this drug after a chest infection had returned an the initial antibiotics had not 100% worked. Within a day I had gotten a lot worse, and began passing out and ended up in hospital for 6 hours. They informed me that this drug was not always reliable and they had a woman in the ward for 6 weeks due to her taking these for 3 days. Reluctantly I came off this drug and went back onto a longer dosage of my initial antibiotics. Needless to say I got a lot better quickly, and the infection had disappeared within a week."