The use of alpha (1 and 2) as well as beta (1 and 2) agonists (stimulants) and antagonists (blockers) may produce profound autonomic nervous system (ANS) effects.  The well known beta-1 blockers have well-demonstrated effects (direct and indirect) on myocardial function (and elsewhere), i.e., inotropic (force of contraction), chronotropic (heart rate), as well as on the rate of spread of electrical conduction within myocardial tissue.  As has been pointed out before on this message board, these profound effects are of little practical concern in a polygraph examination, inasmuch as they are tonic effects (not phasic effects) which will affect the cardiovascular physiology monitored over the totality of the polygraph exam, effecting both control and relevant question responses whose intercomparison is the basis for polygraph scoring.  Remember...the goal of one who attempts polygraph countermeasures is to increase control question responses relative to relevant question responses, not reduce (or increase) all responses equally.  You mention Central Nervous System (CNS) effects (antianxiety)....yes, they do exist, but the same story is likely to exist for these effects as well--they would likely be undifferentiated amongst responses to control and relevant questions.

Not only is such pharmacological intervention largely ineffective (there, however, may be some value in some situations in reducting all responses yielding an inconclusive result which can then be coupled with simply saying "no way" (behavioral countermeasure) to any post-test accusations), but these type of approaches are largely moot because of the very effective physical and mental countermeasures available to accomplish the task...