Mouthwash is a liquid that is used to rinse out the mouth. Some mouthwashes are antiseptic and help reduce the amount of bacteria in the mouth. Other mouthwashes may have analgesic, anti-inflammatory, or anti-fungal properties, or they may have moistening properties that help with dry mouth. Some mouthwashes have only cosmetic properties and are used to control or reduce bad breath. Mouthwash is usually swished around in the mouth and may also be gargled, and some mouthwashes are intended to be swished or gargled for a prescribed amount of time in order to be clinically effective. Fluoridated mouthwashes used in conjunction with fluoride toothpaste may help maintain or improve the amount of available fluoride in the oral cavity.

People have been using mouthwash and mouth rinses for centuries to help curb gum disease and to refresh the oral cavity. These rinses often consisted of medicinal plants; warm salt water rinses have also been used in many cultures to relieve pain in the oral cavity and throat, and they are still widely used today. Early scientific experimentation helped scientists conclude that prolonged exposure to different antiseptic compounds was more effective than quick rinses, which remained the standard wisdom until the introduction of chlorhexidine in the 1960s. Because chlorhexidine adheres to the surfaces inside the mouth, it remains present and active for a sufficient amount of time to effectively destroy bacteria. Since the introduction of chlorhexidine, the number of commercially available mouthwashes has skyrocketed, and well more than a hundred types of commercial antiseptic mouthwash are available today.

The majority of mouthwash is used for antiseptic purposes, as part of an oral hygiene practice. Because mouthwash has no standard formulation, mouthwashes may use a variety of ingredients to treat different conditions. While mouthwash does have some antiseptic and antibacterial properties and may help eliminate the plaque that leads to tooth decay, gum disease, and bad breath, it is important to use mouthwash in conjunction with proper brushing and flossing, as mouthwash alone is not sufficient to maintain the cleanliness and health of the oral cavity and does not eliminate the need for a thorough oral hygiene routine. Additionally, the American Dental Association maintains that brushing and flossing alone, along with professional cleanings and regular dental checkups, are sufficient to maintain the levels of bacterial plaque in the oral cavity, and that mouthwash is not an integral part of an effective oral hygiene routine. Some prescription mouthwashes, however, are an important part of home care for the patients to whom they are prescribed.

Using mouthwash may lead to some minor side effects, including disturbance of the taste buds or a feeling of dryness in the mouth; these common side effects are usually temporary. When users are allergic to any of the ingredients in mouthwash, soreness or ulceration may occur and the irritating ingredient should be avoided. Prescription mouthwash may be prescribed for use before and after oral surgical procedures, or to relieve the pain and inflammation associated with cancer treatments. Mouthwashes may also be prescribed for oral ulcers and mouth pain, or for inflammation of the periodontium.