Propolis or bee glue is a resinous mixture that honey bees produce by mixing saliva and beeswax with exudate gathered from tree buds, sap flows, or other botanical sources. It is used as a sealant for unwanted open spaces are usually filled with beeswax. Its color varies depending on its botanical source, the most common being dark brown. Propolis is sticky at, and above, room temperature (28 C (68F)). At lower temperatures, it becomes hard and very brittle. The National Institutes of Health rates propolis as "possibly effective" for treating cold sores, genital herpes, and post-surgery mouth pain. Currently, there is "insufficient evidence" to rate the effectiveness of propolis in treating other conditions. Propolis is also used to make cough drops for cough and throat irritation.