I've been on 100mg of Doxycycline for 4 days due to Bronchitis. I've experienced dizziness, disorientation, lack of appetite, and profound fatigue. While my Bronchitis symptoms are improving, the dizziness and fatigue are really limiting anything I can do during the day. I did some errands for about an hour and a half today and had to come home and take a 2 hour nap to recover. I've not had this happen to me before while taking meds of this sort.