Expired baking powder can still be used, but it may not be as effective as fresh baking powder. The only danger of using expired baking powder is that it may not properly rise, resulting in baked goods that are flat and dense. To test if expired baking powder is still good, you can mix a teaspoon of baking powder into 1/3 cup of warm water. If it fizzes, its still good and will cause the dough to rise in the oven. If it has a delayed or weak reaction, or simply dissolves or sinks to the bottom of the water, it is expired and should be thrown away. Its best to use baking powder before its expiration date, which is usually 18 to 24 months after manufacture.