Doxycycline Monohydrate Side Effects: I see that of only 2 postings as of this one from me, one menti http://www.medications.com/effect/view/23468 I see that of only 2 postings as of this one from me, one mentions feeling "generalized weakness and fatigue." BINGO! Me too. And...like the posting I'm referring to, the patient is taking it for Lyme Disease. I know that, itself, can also cause weakn I took doxycycline for Lyme Disease also and was in bed for 5 day Sat, 22 Sep 2007 14:50:49 -0400 http://www.medications.com/effect/view/23468?item=1984 I took doxycycline for Lyme Disease also and was in bed for 5 days and slept most of the time and was very sick until I was taken off it and switched to another antibiotic. http://www.medications.com/effect/view/23468?item=1984