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Comparing Alpha-2-Adrenergic Agonists

In this article, we take a look at the Alpha-2 Adrenergic Agonists. In comparing Alpha-2 Adrenergic Agonists, we will primarily look at dosing, adverse effects, and notable indications.

Clonidine is FDA-approved for attention deficit hyperactivity disorder (ADHD), cancer pain, and hypertension.  It is also used off-label for a variety of reasons, such as hot sweats, opioid withdrawal, chronic pain, restless leg syndrome, spasticity, and violent behavior.  Clonidine is a centrally acting alpha 2 adrenergic agonist which reduces sympathetic outflow from the CNS, thus reducing peripheral resistance, renal vascular resistance, heart rate, and blood pressure.  Dosing clonidine depends on the indication, but patients will typically take clonidine by mouth in doses of less than 1 mg 2-4 times per day. When used for hypertension, patients typically take 0.1 to 0.3 mg twice daily.  Clonidine is also available as a weekly transdermal patch that releases 0.1 mg, 0.2 mg, or 0.3 mg per day. 

Guanfacine is FDA-approved for ADHD (either as monotherapy or in addition to a stimulant) and hypertension.  It can also be used off-label for Tourette’s syndrome, opioid withdrawal, or renovascular hypertension.  Guanfacine is a centrally acting alpha-2 adrenergic receptor agonist that acts similarly to clonidine.  Guanfacine is more selective than clonidine, and it does not inhibit dopamine turnover as clonidine does.  Neither clonidine or guanfacine are CNS stimulants, and their mechanism of action for ADHD is unknown.  Pediatric patients with ADHD may take 1 mg of extended-release guanfacine per day or a weight-based dose of the immediate-release formulation.  Adults with hypertension may start with a dose of 0.5 mg – 1 mg at bedtime, and they may need to titrate up to 2 mg after 3-4 weeks.

Clonidine (podcast) and guanfacine share many potential adverse effects. Patients taking clonidine or guanfacine may experience orthostatic hypotension, abdominal pain, constipation, dry mouth, dizziness, confusion, insomnia, or fatigue.

Methyldopa is FDA-approved for hypertension, and its well-known indication is hypertension in pregnant women because it is considered safe.  It also may be used off-label for menopausal hot sweats.  Methyldopa acts on alpha-adrenergic receptors to decrease heart rate, peripheral resistance, and blood pressure. This medication significantly lowers plasma renin activity.  Potential adverse effects from methyldopa include asthenia (weakness), dizziness, headache, erectile dysfunction, hepatotoxicity, or heart block.  When pregnant women take methyldopa for hypertension, they may start at 250 mg 2-3 times per day and titrate as needed.

Tizanidine is FDA-approved for spasticity, and it is used off-label for headaches and acute pain.  Although tizanidine is also an alpha 2 adrenergic medication, it is notably different from the other three medications discussed. Tizanidine is not used for hypertension, though it does have mild, transitory blood pressure lowering effects.  Studies have found that tizanidine has just a small fraction of the potency of clonidine when it comes to lowering blood pressure.  Even so, hypotension is an adverse effect to be aware of with patients taking tizanidine. Other common adverse effects to be aware of with tizanidine include xerostomia (dry mouth), asthenia (weakness), dizziness, or tiredness.  Tizanidine is usually taken as needed up to three times per day, starting at 2 mg per dose, with a maximum dose of 36 mg per day. 

This article (Comparing Alpha-2 Adrenergic Agonists) was written by Sarah Jackson, PharmD Candidate in collaboration with Eric Christianson, PharmD, BCGP, BCPS

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