Nitrofurantoin
"I was prescribed Genfura (Nitrofurantoin) after a previous antibiotic was ineffective in curing a UTI. UTI symptoms were completely gone within 3 days, but I had major side effects within hours of taking the first pill.

Side effects: 

Worst flu like symptoms of my life and I nearly went to A&E I was so sick. I had a high fever for 2 days, chills, shallow laboured breathing, no appetite, nausea, constipation, extreme weakness, severe headache and I couldn't leave my bed for 2 days. 

Strangely the side effects went away as soon as they came on, and 4 days into the medication I feel fine."