Abstract: An intra-oral device and a method of using the device to assist overweight persons in treating obesity by reducing and/or limiting the mouth opening and/or slowing down the rate of chewing. Such a device is handled by the user with or without the need for professional help, by increasing the awareness of the user about the need to reduce the amount of food intake in order to lose weight. The intra-oral device is intended to train the user to reduce the mouth opening. The intra-oral device is a restraining device which is anchored in the upper and lower jaws. The restraining device is an elastic element which is inserted between two respective inter-proximal spaces (IPSs) of the upper and the lower jaws of the user. The device reduces and/or limits the mouth opening and/or slows down the rate of chewing. The device is disposable, intended for a single use, and is removable after use. The device can be disengaged in case of a need to open the mouth above the limits of the device.