The expiry date is not indicated for secondary reference substances because the substances comply, where applicable, with the requirement of the pharmacopoeial monograph and are monitored regularly according to the re-testing programme. The issuing body should have effective means of communication to inform users of the validity of reference substances. It is recommended that only an amount sufficient for immediate use be purchased, and that the substances are used as soon as possible. Once the container has been opened efforts should be made to avoid possible degradation, contamination and/or introduction of moisture and/or exposure to air.