I use mine even when it's past date. My understanding is that you may just not have as many "live" critters but I don't think there's any harm in using it.
I have always used this one and others beyond their expiration date. The key to this is to shake each bottle or package well a couple times a day. It will last quite a long time after it's expiration. You will know by the smell if it's bad.
Hello, when the OceanMagik dies/ expires it will develop a strong sulfur smell. If a sulfur smell is present the item should be discarded. We do not recommend using the product after the expiration date, if you choose to use the item after the expiration date you do so at your own risk and care should be taken to ensure that the product has not died and is still alive. Thank you!