Learning Objective
By the end of this section, learners should be able to identify α₂-adrenergic agonists, explain their mechanism of action, and correlate their clinical uses with characteristic adverse effects, with emphasis on USMLE Step 1–relevant pharmacology concepts.
α₂-agonists act centrally to decrease sympathetic outflow by inhibiting norepinephrine release from presynaptic neurons. This results in reduced heart rate, blood pressure, and overall sympathetic tone.
Drugs, Uses, and Adverse Effects
Clonidine, Guanfacine →
Hypertensive urgency (selected cases), ADHD, Tourette syndrome, opioid withdrawal symptom control
Adverse effects → CNS depression, bradycardia, hypotension, respiratory depression, miosis; rebound hypertension with abrupt withdrawal
α-Methyldopa →
Hypertension in pregnancy
Adverse effects → Positive direct Coombs test with hemolytic anemia, drug-induced lupus, hyperprolactinemia
Tizanidine →
Relief of muscle spasticity
Adverse effects → Hypotension, muscle weakness, xerostomia (dry mouth)
