Centrally Acting Alpha2 Agonists

Centrally Acting Alpha₂ Agonists

Centrally acting alpha₂ agonists are another category of drugs that treat hypertension. They are also approved for severe pain relief (epidural injection) and management of attention deficit hyperactivity disorder (ADHD).

Prototype and Other Medications

The prototype drug for centrally acting alpha₂ agonists is clonidine, also called Catapres. Other centrally acting alpha₂ agonist medications are methyldopa and guanfacine. 

Expected Pharmacologic Action

Centrally acting alpha₂ agonists exhibit their antihypertensive effects by selectively activating alpha2 receptors in the brainstem areas of the central nervous system (CNS). This results in a reduction of sympathetic stimulation of the heart and associated blood vessels, thereby reducing blood pressure.

Adverse Drug Reactions

Centrally acting alpha₂ agonists can cause CNS effects such as drowsiness and dizziness, as well as xerostomia (dry mouth). Rebound hypertensive crisis may result if the client abruptly discontinues clonidine.

Interventions

When administering centrally acting alpha₂ agonists, monitor client for CNS effects, such as drowsiness and dizziness. Tell clients that dryness of the mouth will decrease over the first few weeks of therapy. If the provider is working with a client in whom a centrally acting alpha₂ agonist is being discontinued, be sure the prescription indicates that the dose must be tapered slowly, over a period of several days.

Administration

Clonidine, the prototype for the centrally-acting alpha₂ agonists is available in an oral form, as well as a transdermal patch when the provider gives it to treat hypertension. Give an oral dose at bedtime to prevent daytime sedation. Begin oral dosage low, and gradually increase to prevent severe hypotension. Apply transdermal patch to a dry, relatively hairless area of the skin on the upper outer arm, or anterior chest, once every 7 days. Rotate transdermal patch sites and monitor skin for inflammation or irritation. Be sure to remove the old patch before applying a new patch.

Client Instructions

Tell clients to take their centrally-acting alpha₂ agonist at bedtime to minimize CNS effects. Warn them against performing hazardous activities such as driving until the drug’s effects are known. For dry mouth, recommend to clients that they suck on hard candy, chew sugarless gum, or sip water to minimize this effect. Assure clients that dry mouth diminishes with time. Tell clients not to stop taking clonidine abruptly. When the provider discontinues its use, taper clonidine according to instructions.

Safety Alert

Clonidine is a centrally acting alpha₂ agonist, which means the target receptors are in the CNS; more specifically in the brainstem. By stimulating these receptors, sympathetic outflow to the peripheral blood vessels is significantly decreased, causing the blood vessels to passively dilate and the heart rate to slow. It is for this reason that stopping clonidine is unsafe. When stimulation of the alpha2 receptors re-establishes, rebound outflow from the sympathetic nervous system will occur, causing a sudden and significant rise in blood pressure. Clients need to understand that when stopping the drug, they need to taper it over a period of several days to avoid a hypertensive crisis.

At high doses, clonidine can result in feelings of euphoria, sedation, and hallucinations, which is desirable to some clients, especially those abusing cocaine or opioids. Monitor clients for appropriate use of clonidine to ensure they are not abusing the medication.

Contraindications and Precautions

Don’t give the centrally-acting alpha₂ agonist, clonidine, to clients who are on anticoagulant therapy. Use centrally-acting alpha₂ agonists with caution in clients who have severe cardiac disease, cerebrovascular disease, or renal disorder.

Interactions

The centrally-acting alpha₂ agonist, clonidine, increases the sedation effects of CNS depressants, including alcohol, antihistamines, and opioid analgesics, etc. Antihypertensive effects can be decreased when concurrently administered with MAO Inhibitors, tricyclic antidepressants, or amphetamines.