Taking methylcobalamin has a rash, belongs to the allergy caused by the drug rash, usually about 1 week after stopping the drug recovery, recovery time according to personal constitution, absorption of the drug and other factors vary, the recovery time of the serious cases need to be longer. Mecobalamin is mainly used for peripheral neuropathy. Some patients may experience allergic reactions after taking the drug, such as skin rashes all over the body, accompanied by itching and other symptoms. At this time, the drug needs to be stopped immediately, and for mild symptoms, recovery usually takes about 1 week after stopping the drug. When the itchy rash is severe, topical glycerite lotion, glucocorticoid cream (hydrocortisone butyrate, dexamethasone) can be used to stop itching, along with oral antihistamines, such as loratadine, cetirizine and so on. More severe symptoms, in addition to the rash is accompanied by dyspnea, abdominal pain, and even shock, need to immediately go to the hospital rescue, systematic use of glucocorticoid drugs desensitization treatment, such as intravenous dexamethasone. Its recovery time is long, often taking weeks or even months. The appearance of drug rash, the need for professional physicians under the differential diagnosis of whether the drug rash, so as to treat the symptoms of the drug, not unauthorized diagnosis, abuse of drugs. The drugs mentioned above should be used according to medical advice.