Doxycycline 100mg injection is an intravenous formulation of the tetracycline antibiotic used when oral therapy is not feasible, covering respiratory, urogenital, skin, and atypical/zoonotic infections, and for specific indications like anthrax post-exposure prophylaxis; it is a lyophilized doxycycline hyclate powder reconstituted for IV infusion and typically transitioned to oral therapy as soon as possible. It is for IV use only; rapid administration should be avoided due to thrombophlebitis risk, and solutions require protection from light and timely use after reconstitution per labeling. Clinical use follows culture/susceptibility and guideline-based selection of antimicrobial therapy.