I have only taken 3 - 100mg doses and that's enough for me. The doctor prescribed it for a sinus and bronchial infection. I thought I was just getting much sicker but now know it is the doxy after reading the posts. I have serious nausea, headache, heartburn, feeling like something is stuck in my throat, upper back pain and all day I felt like I couldn't keep my eyes open. I didn't feel half this bad when I went to the doctor. I will call her tomorrow and ask for something else. I have never had a reaction to any antibiotic other than an occasional yeast infection. Thanks for all your posts!