clonidine
Meaning of clonidine: An old blood pressure drug also used for ADHD, tics, hot flashes and opioid withdrawal, all through the same calming action on nerve traffic.
Definition of clonidine
Clonidine is an alpha-2 adrenergic agonist, a medicine acting on receptors in the brainstem to reduce the outgoing nerve traffic that keeps blood vessels tight and the heart beating fast. Lowering that traffic lowers blood pressure. The same damping action on the sympathetic nervous system underlies its other uses, which reach well beyond blood pressure.
How does it lower blood pressure?
The sympathetic nervous system is the branch that readies the body for exertion: it narrows blood vessels, speeds the heart and raises pressure. Its outgoing signals originate in the brainstem. Clonidine crosses into the brain and stimulates alpha-2 receptors there, and those receptors act as a feedback brake, cutting the release of norepinephrine and so quietening the traffic leaving the brainstem. Vessels relax, the heart slows, and pressure falls. The word agonist repays a pause, because it is easily misread. An agonist switches a receptor on, and clonidine switches on a receptor whose job is to suppress, so activating something produces a damping result. That is a different mechanism from an alpha-adrenergic antagonist, which blocks the receptors that tighten vessels directly.
What else is it used for?
| Use | What it addresses |
|---|---|
| Attention deficit hyperactivity disorder | Approved in extended release form, alone or alongside a stimulant |
| Tic disorders, including Tourette syndrome | Reduces how often and how forcefully tics occur |
| Hot flashes | An option where hormonal treatment is unsuitable |
| Opioid withdrawal | Damps the sympathetic surge behind sweating, cramps and agitation |
| Severe cancer pain | An injectable form is given into the epidural space in some countries |
| Anesthesia and sedation | Added to other agents to deepen and prolong their effect |
Why does stopping it matter?
The sympathetic system adapts to being suppressed. Receptors grow more responsive during continued treatment, so releasing the brake abruptly leaves a system that has become more sensitive than it was, and blood pressure can rebound above the level it started from, accompanied by agitation, sweating, headache and a racing heart. The effect is well described and is the reason a clonidine dose is reduced in steps rather than halted. Its other effects follow from the same mechanism: drowsiness, dry mouth, a slow pulse, and dizziness on standing because the reflex tightening of vessels is blunted. A skin patch formulation holds the level steadier than tablets and is one reason it is used where swallowing is difficult. Because it is frequently prescribed for reasons unrelated to high blood pressure, seeing it on a medicine list says little on its own about why it was started.
Also known as
Used in a sentence
Clonidine was included in the patient's medication regimen for blood pressure control.
Good to know
Clonidine is an alpha-2 adrenergic agonist acting in the brainstem to reduce sympathetic outflow, a mechanism distinct from medicines that act on the vessels or kidneys directly, and abrupt withdrawal after regular use can send blood pressure above its pretreatment level.