I took doxycycline for a few years to deal with acne. I developed acid reflux as a result, and managed to get myself horrifically sunburned once or twice. Wouldn't suggest taking it as a preventative measure.
I also took doxycycline for acne for months and it messed up my gut big time, took years to return to normal. It also gave me horrible nausea. Really don't think it should be prescribed for acne, especially because long-term antibiotic usage encourages resistance. I've now been on Roaccutane for over 6 months and couldn't be happier with the results.
Yeah this is an absolutely terrible idea. Antibiotics should be avoided unless needed, they will wreck your digestive system.