Thanks for contacting Ask Extension. Once a canning lid is subjected to vacuum sealing is it considered used. The Extension Service recommends that canning lids not be reused. The sealing compound on the lid may get worn down (indented) and not seal a second time, and unsealed jars pose a food safety risk. You can reuse canning lids to store dry items, however. According to the National Center for Home Food Preservation, the sealing compound on canning lids works well for 5 years from the manufacturing date. Once that date is exceeded, the lids may fail to seal, again posing a food safety risk.