I had took my first doxycycline pill for my acne problem, I immediately felt a tingling upset stomach, but it was just that. The next morning after breakfast, I took another pill and felt heavy in my head. I vomit all the food. I had taken the pill for 3 days and I found a severe headache and tiredness all the day. But, i think i will continue the pill for another 10 days. Hopefully won't come out big problems.