The polygraph works by recording changes in a person's Sympathetic
Nervous System, part of the Autonomic Nervous System, which operates independently
of conscious thought. For example, your lungs and heart continue to operate
even when you are asleep - you don't have to think about it. These systems can
be consciously controlled only very slightly, and attempts to change these systems
are usually picked up by the examiners, who are trained to identify such things.
It is highly unlikely that someone can alter the outcome of a polygraph exam,
but it is not impossible. A verified accuracy rate as high as 95% attests to
Many examiners are now using "countermeasures detection" equipment which easily identifies anyone attempting to use the techniques taught by some web sites and government agencies. unfortunately, many honest people are found "deceptive" to the test questions after attempting to use these techniques simply because they attempted to influence their test results.
Use of certain drugs and medications can also affect the exam, but such use generally results in an "inconclusive" test. It is virtually impossible to change a result from "deceptive" to "truthful" through the use of drugs or medications prior to an exam. If drugs are suspected, a pre-test (or post-test) drug screening is advised.