Dysgeusia is a usually unpleasant phantom taste that is persistent in the absence of any external stimuli that would cause the taste. While it is possible for certain substances, such as medication or molecules present within the blood, to move into the salivary glands and produce the gustatory sensation, a true dysgeusia is demonstrated only when the experience is shown to be linked directly to the central nervous system. This is tested by administering an anaesthetic to the mouth, which blocks signals originating in the mouth from reaching the brain. It is common in cases of dysgeusia for the anaesthetic to not only fail in blocking the taste, but also to increase the perceived intensity of the taste. Since dysgeusia is neuronal in origin, it is caused by pathology in the central nervous system such as disease or tumours.