The British Medical Association says certain mouthwashes contain chemicals that kill bacteria and there’s good evidence that they work. Some studies suggest bad breath can improve after using an antibacterial mouthwash for two to four weeks. There is some benefit from one-time use but it only usually lasts a few hours.
Of course, bad breath can be caused by other things, such as eating garlic and other fragrant foods, as well as poor dental hygiene. Certain medical conditions also can cause bad breath. In these cases, a mouth rinse may help mask your bad breath, but it won't address the cause of it.