Clear crystal blisters on the inside of the mouth may be caused by mucous gland cysts, viral herpes diseases, and infections. 1. Mucous gland cysts: usually occurring on the lower lip, caused by oral ulcers or bite wounds. After injury to the submucosal glands, the ducts of the mucous glands of the lower lip will atrophy and bend, making it difficult to discharge mucus. When the gland’s secretion is blocked, a cyst will form and a large translucent crystal vesicle will appear. Generally, the texture is soft and elastic. 2. Viral Herpes Disease: Viral herpes disease may also cause clear crystal vesicles on the lining of the mouth. Generally for blistering herpes in the mouth and lips, patients can take antiviral medications such as ribavirin and interferon under the guidance of a doctor. 3. Infection: In general, when the mouth is infected by fungi, the inner wall of the mouth may also develop clear crystal blisters. There are many other causes of clear crystal blisters on the inner wall of the mouth, and patients are advised to seek timely medical treatment under the guidance of a doctor to avoid delaying the condition.