The main reason why you can’t open your mouth when eating betel nut may be due to fibrosis of the oral mucosa or ankylosis of the temporomandibular joint. 1. Betel nut is hard, long-term chewing will cause continuous wear and tear on the oral mucosa, making the oral mucosa fibrosis and elasticity deteriorate; ultimately, the mucosa hardens to form strips, resulting in the mouth can not be opened and closed properly. 2. Long-term chewing of hard objects such as betel nut will also aggravate the burden on the temporomandibular joints, resulting in organic lesions of the joints, i.e., ankylosis of the temporomandibular joints, which is clinically characterized by difficulty in opening the mouth and is accompanied by pain and other symptoms. First of all, it is not advisable to chew betel nut and other hard objects for a long time, and secondly, it is recommended to consult a doctor for symptomatic treatment to avoid delaying the condition and even aggravating the symptoms.