Dr. Philip Tierno says most plastics today are biodegradable. When it comes to re-using a plastic bag, it depends on what was in the bag first. If you have meat, meat products or cheese in the plastic bag, throw it away after one use. You may use the bag more than once if fresh produce was in it first—provided you thoroughly wash and dry the bag. Throw the bag away if the food previously in it was moldy or if there are any holes or tears in the bag.