I started taking this med for treatment of acne. Doxycycline 100 MG twice daily. Within 4 days, I started experiencing chest pain, arm and back pain, palpitations, dizzieness, weakness in the legs and arms, diarrea, pain in the eyelids. I thought i was dying! I followed all of the instructions from the pharmacist and the doctor, and it still happened. They tried to tell me it was a problem with anxiety since it runs in my family. After finding this site, i think differently. I think anxiety might be a part of it, but not what caused it, i will never take tetracycline based products again, and will never recommend them to anyone.