Doxycycline + lactic acid bacillus is a commonly marketed combination medicine that pairs a broad-spectrum antibiotic (doxycycline) with a probiotic (typically a Lactobacillus or other lactic-acid-producing bacillus). The aim of the combination is to treat susceptible bacterial infections while reducing the risk of antibiotic-associated gastrointestinal side effects such as diarrhea and dysbiosis.