How do I use a life gem?

[ChugMonkey]

I tap my bottom arrow about 10 times per second and yet it still doesnt use my lifegem how do I use it?

[Senrei]

Hit X to use items, if using a controller.

[zero]

or E on pc

[Johnny The Fox]

On a keyboard, default is "E" ("Down Arrow" is just to select the item).