General eye drops can be used within one month after opening. Eye drops after opening, contact with the external environment, may be contaminated and deteriorated, equivalent to the preservation of the environment has changed, the efficacy of the drug will also have certain changes. After the eye drops are opened, they cannot be reused even if they are not used up after one month. Eye drops in order to extend the shelf life, in which a certain concentration of preservatives added, preservatives themselves will have a certain impact on the eyes, so the concentration of preservatives will not be very high, more secure, safe concentration of preservatives so that the ability to extend the life of the eye drops is limited to a long time to open the seal is still contaminated. So eye drops are safer to use within one month after opening, specifically consult a medical professional for advice.