Most general anesthetics, which are used to put patients to sleep during surgery, can actually increase the discomfort patients feel when they wake up.

"Noxious" anesthesia drugs -- a category which includes most general anesthetics -- activate and then sensitize specific receptors on neurons in the peripheral nervous system.

It was already known that general anesthetics cause irritation at the infusion site or in the airways when inhaled, and that they can activate pain-sensing nerve cells in the peripheral nervous system. However, the specific mechanism by which anesthetics affect sensory neurons was not known until now, nor the fact that anesthetics can continue to cause pain and inflammation even as they're used during surgery.