How Safe Are Doxycycline And Lactic Acid Bacillus Capsules In A Long Term?

I have been told to have doxycycline and lactic acid bacillus capsules for 30 days and now have been asked to extend it for another month. I am undergoing a treatment for 'hair follicle'/hair root infection which has gone to a extreme stage. How safe is it to have such anti-biotics for such a long period? what side effects will it have in long terms. My hair follicle and hair root infection sizes have reduced drastically but its still there. Is it safe to take doxycycline and lactic acid bacillus as i am also doingweight training and taking protein supplements and caffiene pills. Please advise