I am taking Doxycycline 100mg 2 times a day for 3 weeks for lyme. the only side effect that I did have was extreme nausea and stomach pain. it happened 3 times and all 3 times I only drank a small glass of water. I found that drinking 16 oz or more would have no side effects. I feel great and still do all activities with absolutely no side effects.i have 3 more days till Im done.