Is it safe to use cephalexin 500/twice a day against acne for a period of 6 months.should i use any biotin at least?

Safe is relative: It is a utilized therapy for acne, as are several other oral antibiotics. But anytime you undertake long term antibiotic use, there is a chance of an adverse effect like antibiotic associated diarrhea, c.diff colitis, alterations in the natural bacteria of your body, resistant microbes. With that said, it's still rather safe.