For what's it worth, before i was into this health stuff, i was prescribed doxycline for acne by a derm. I took it for 2-3 months. After 2nd month , it was as if my gut tore apart. Started having diarrhea and fatigue ( i had the digestion of steel prior to this event so i couldn't identify what was the cause). Didn't know mass bacterial killing can cause opportunistic bacterial overgrowth. Went to a gastro MD, was prescribed VSL#3 (bifidobacteria based probiotic) ,took around 2-3 packs (a single pack contains 10 tablets ) and felt great relief . You can search VSL#3 on pubmed . It has decent research backing it and it's patented tech makes sure the germs reach colon ( atleast that's what they say).