<s>[INST] Hello my boyfriend has just discovered what appears to be a blood blister on the underside of his tongue, it appeared this evening and he could feel it swelling for a couple hours. It has popped now, releasing a little blood, although it s still sore he hasnt had anymore problems. I was wondering if you could tell me what causes blood blisters? [/INST] Thanks for your query, I have gone through your query. The blood blister what you are describing can be a hematoma secondary to any trauma to the floor of the mouth or tongue. It can be a manual also. It is a cystic lesion occurring secondary to trauma to salivary glands. Nothing to be panic, consult an oral physician and get it evaluated. If it is a hematoma, nothing to worry, do hot and cold water rinse, do not traumatize that area. If it is a manual consult a maxillofacial surgeon and get it excised. Do saline gargling. I hope my answer will help you, take care. </s>