An adjustable mandibular advancement device has an upper plate for covering upper teeth of a human and a lower plate for covering lower teeth of the human. A magnetic closure mechanism is provided in the device in order to hold the upper plate in contact with the lower plate when the device is positioned in a persons mouth. The magnetic closure mechanism includes one or more magnets in the upper plate and one or more magnets in the lower plate. The magnets are positioned so that when the upper plate and lower plate are in contact, the lower plate pulls the persons lower teeth and lower jaw forward and down in order to reduce obstruction of an airway during sleep or physical activity such as sport. An adjustment mechanism between the plates allows the position of the lower plate relative to the upper plate to be adjusted.
