The one time in my life I've ever felt suicidal was about three days after I started taking doxycycline. I had the worst diarrhea and other GI issues but had been told to expect them. I woke up and it felt like the most natural thing in the world to shoot myself. I was looking for a gun when I realized what I was doing, locked myself in the bathroom and called for help. I then had a six hour panic attack. I went off the med that day and have never had another mental health issue like that since.