Olopatadine hydrochloride is an antihistamine, mainly used for the treatment of allergic diseases, and the exact number of days to take the drug should be determined by the doctor’s prescription and according to the changes of the disease. There is no definite course of treatment, usually the rash is completely resolved and the medication should be continued for 3-5 days. For chronic diseases, it may be necessary to use the medication for a long period of time, gradually reducing it after improvement, intermittent administration until it is discontinued, and sometimes it may be necessary to combine it with other chemically structured antihistamines to treat allergic diseases. If you have an allergic rash, do not self-medicate, but go to a regular hospital and follow the doctor’s instructions for regular medication, and then gradually reduce the medication or choose to stop it after regular follow-ups.