Nitrofurantoin
"I had a UTI and my Dr. prescribed Macrobid.  I'm allergic to most antibiotics and usually experience all the side effects possible so I read the reviews here and was so scared to take it that I waited a week to see if my symptoms would somehow go away on their own (they didn't).  I finally said a prayer and started the Macrobid.  The only side effect I had at all was a pretty good headache on days 1 and 2, which was relieved with Advil.  I wish I had started sooner!  Just remember that usually, the only people coming to these sites are people who have had bad reactions.  My pharmacist said that she's been prescribing it for 35 years and never heard of anyone having really bad reactions to it."