There are many causes of 8mm effusion in the left pleural cavity, such as pneumonia, pleurisy, tuberculosis, etc. Different treatments should be chosen according to the causes. 1. Pneumonia: patients suffering from pneumonia will have cough, fever, chest pain, increased pleural effusion and other symptoms, caused by external bacterial and viral infections, it is recommended to use medication under the guidance of a doctor, such as cefradine capsules, cefamandole tablets, etc., which can alleviate the symptoms. 2. Pleurisy: due to the invasion of bacteria, fungi, or trauma, can cause pleurisy disease, chest pain, chest tightness, pleural effusion increase and other symptoms, can be under the guidance of the doctor to use medication, such as amoxicillin capsule, cefuroxime, etc., can alleviate the symptoms. 3. Tuberculosis: Tuberculosis is caused by tuberculosis bacillus, which enters the body through respiratory transmission and leads to infection, and symptoms such as fever, cough and pleural effusion will appear, which can be treated with medicines under the guidance of doctors, such as rifampicin tablets, pyrazinamide capsules, etc., which can alleviate the symptoms. It is recommended to go to the hospital in time to check and cooperate with the treatment, and not to use drugs blindly by oneself.