Doxycycline Hyclate Side Effects: I was prescribed doxycycline 2xday for seven days. On the third http://www.medications.com/effect/view/30712 I was prescribed doxycycline 2xday for seven days. On the third day I started to feel very sick. However, it was not until the 5th/6th day that I began to suspect I was having a reaction to the antibiotic. I have never had a negative reaction to any medi Wonder if your reaction to the antibiotic could have been what is Fri, 23 May 2008 10:40:15 -0400 http://www.medications.com/effect/view/30712?item=8552 Wonder if your reaction to the antibiotic could have been what is called a Herxheimer effect. Herxheimer is when a person actually has some bacteria in their system, and the antibiotic begins to kill the bacteria, and the person feels terrible due to the http://www.medications.com/effect/view/30712?item=8552