Nitrofurantoin
"I take this medicine for urinary tract infections during pregnancy.  I am on it now again for the third time.  Take twice a day for seven days.  The side effect I get comes right after the first pill.  I get flu like symptoms: sore joints, chills, sleepiness and extreme weakness.  However, no fever.  Took one Tylenol and sweated it out while sleeping woke up better.  But, this medicine works really well after the first dosage."