- Rifaximin is an antibiotic with a broad-spectrum activity against gram-positive and gram-negative microorganisms, both aerobes and anaerobes within the gastrointestinal tract. The main advantage of rifaximin is that it is poorly absorbable, which minimizes the antimicrobial resistance and adverse events and renders the drug safe in all patient populations. In addition, rifaximin has a better activity against gram-positive organisms than norfloxacin.