lyme disease
"Work miracles to keep Lyme in check, does not necessarily eradicate it.  Took about 4 years since infection for Lyme to get a foothold before I was able to get proper medical attention.  On badly spread lyme, expect severe side effects -- much of it, so I have been told -- have to do with chemicals resulting from the bacteria dying rather than the doxycycline itself: Severe depression and suicidal thoughts that disappeared as soon as I finished the medication (discomfort, but at least I was able to put up with it knowing it would pass once the six-week 500mg/day was up).

Also, take with proper meals -- I've woken up to throw up blood when taken in empty stomach.  Also, stay out of direct sunlight, or, if possible, take during winter months"