Skip to Main Content


Hypertension is one of the most common chronic medical problems and one of the most readily amenable to pharmacotherapy. Beginning in the 1960s, when asymptomatic hypertension was linked to significant adverse effects such as stroke and sudden death, antihypertensive pharmacotherapeutics began being used. The first generation included centrally acting, sympatholytics, direct vasodilators, sodium nitroprusside, and diuretics. Unfortunately, these often had significant side effects, leading to the development of not only β-adrenergic antagonists and calcium channel blockers (CCBs) and more recently, angiotensin-converting enzyme inhibitors (ACEIs), and angiotensin receptor blockers (ARBs). This chapter reviews the first-generation antihypertensives, as well ACEIs and ARBs. In general, the majority of antihypertensives manifest clinical signs and symptoms in terms of the degree of hypotension produced. Particular attention will be placed on mechanisms of action and unique toxicologic considerations for each of these xenobiotics.


Clonidine is an imidazoline compound that was synthesized in the early 1960s. Because of its potent peripheral α2-adrenergic agonist effects, it was initially studied as a potential topical nasal decongestant. However, hypotension was a common side effect, which redirected its consideration for other therapeutic applications.95 Clonidine is the best understood and the most commonly used of all the centrally acting antihypertensives, a group that includes methyldopa, guanfacine, and guanabenz. Although these drugs differ chemically and structurally, they all decrease blood pressure in a similar manner—by reducing the sympathetic outflow from the central nervous system (CNS). The imidazoline compounds oxymetazoline and tetrahydrozoline, which are used as ocular topical vasoconstrictors and nasal decongestants, produce similar systemic effects when ingested95 (see Chap. 50).


Since 1985, the increased efficacy and improved side effect profiles of the newer antihypertensives have diminished the use of the α2-adrenergic agonists in routine hypertension management. However, clonidine use is increasing as a result of a wide variety of applications, including attention-deficit hyperactivity disorder (ADHD); peripheral nerve and spinal anesthesia; and as an adjunct in the management of opioid, ethanol, and nicotine withdrawal.110,116,120,202 In addition, abuse of clonidine may be a growing problem in opioid-dependent patients, and it has been used in criminal acts of chemical submission.17,134


Although centrally acting α2-adrenergic agonist exposure is relatively uncommon, it may cause significant toxicity, particularly in children. One report from two large pediatric hospitals identified 47 children requiring hospitalization for unintentional clonidine ingestions over a 5-year period.241 Significant clonidine poisoning has also resulted from formulation and dosing errors in children.189,219 Imidazolines used as ocular vasoconstrictors have resulted in significant systemic toxicity as well.95,138,126,180




Clonidine and the other centrally acting antihypertensives exert their hypotensive effects primarily via stimulation of presynaptic α2-adrenergic receptors in the brain.174,196,233 This central α2-adrenergic receptor agonism enhances the activity of inhibitory neurons in the vasoregulatory regions of the CNS, notably the nucleus tractus solitarius ...

Want remote access to your institution's subscription?

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.


About MyAccess

If your institution subscribes to this resource, and you don't have a MyAccess profile, please contact your library's reference desk for information on how to gain access to this resource from off-campus.

Subscription Options

AccessEmergency Medicine Full Site: One-Year Subscription

Connect to the full suite of AccessEmergency Medicine content and resources including advanced 8th edition chapters of Tintinalli’s, high-quality procedural videos and images, interactive board review, an integrated drug database, and more.

$595 USD
Buy Now

Pay Per View: Timed Access to all of AccessEmergency Medicine

24 Hour Subscription $34.95

Buy Now

48 Hour Subscription $54.95

Buy Now

Pop-up div Successfully Displayed

This div only appears when the trigger link is hovered over. Otherwise it is hidden from view.