A flexible check valve is introduced within a fluid path for leakage protection. The flexible check valve can be a duckbill/crossbill valve comprising multiple flexible members that restrict the flow of fluid in a tubular member. The flexible check valve is preferably used in conjunction with a drinking straw to prevent liquid from exiting a container such as a drink box. Preferably, the flexible check valve is a crossbill valve that is attached using an adapter. Liquid is designed to only flow through when an external compressive force (e.g. users fingers or lips) is applied to the valve. When force is no longer applied to the valve section, the valve returns to its normally closed position and fluid is wholly or substantially prevented from exiting.