Preventatively using antibiotics is a horrible idea, for one it increases the risk of creating resisitant strains, and we are already running out of antibiotics (especially broadband ones). Moreover, antibiotics in general are known to mess with you gut biome whose importance we are just beginning to understand (we know it plays a role in many physical and mental illnesses for example). Then there are the side effects which for doxycyline include diarrhea, increased risk of bowel cancer, higher sensitivity to the sun (and associated risk of skin cancer).
I don't mean to be "that guy", but "military grade" means nothing nowadays. It can mean anything from "made to exactly standards" to "made by the lowest bidder, and likely to fail the minute it's used".