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Clonidine is a medication used to treat several medical conditions. It is a direct-acting α2 adrenergic agonist.
It has been prescribed historically as an antihypertensive agent. It has found new uses, including treatment of some types of neuropathic pain, opioid detoxification, sleep hyperhidrosis, anaesthetic use, and off-label, to counter the side effects of stimulant medications such as methylphenidate or amphetamine. It is becoming a more accepted treatment for insomnia, as well as for relief of menopausal symptoms. Clonidine is increasingly used in conjunction with stimulants to treat attention-deficit hyperactivity disorder (ADHD), for which it is administered in late afternoon or evening for sleep, and because it sometimes helps moderate ADHD-associated impulsive and oppositional behavior, and may reduce tics. Clonidine can be used in the treatment of Tourette syndrome. Clonidine is also a mild sedative, and can be used as premedication before surgery or procedures. 
Clonidine treats high blood pressure by stimulating α2 receptors in the brain, which decreases cardiac output and peripheral vascular resistance, lowering blood pressure. It has specificity towards the presynaptic α2 receptors in the vasomotor center in the brainstem. This binding decreases presynaptic calcium levels, and inhibits the release of norepinephrine (NE). The net effect is a decrease in sympathetic tone.
Indications & preparations
Clonidine tablets and transdermal patch
Clonidine is typically available as tablets (Catapres, Dixarit), as a transdermal patch (Catapres-TTS), or as an injectable form to be given epidurally, directly to the central nervous system.
FDA approved uses
This medication may also be used to ease withdrawal symptoms associated with the long-term use of narcotics, alcohol and nicotine (smoking). In addition, clonidine has also been used for migraine headaches, hot flashes associated with menopause, and attention deficit hyperactivity disorder.
Clonidine is regularly prescribed to opiate addicts to help alleviate their withdrawal symptoms. It is mainly used to combat the sympathetic nervous system response to opiate withdrawal, namely tachycardia and hypertension, in the initial days of withdrawals. It helps take away the sweating, hot/cold flashes, and general restlessness. The sedation effect is also useful although its side effects can include insomnia, thus exacerbating an already common feature of opiate withdrawal.
Clonidine is used off-label to treat psychiatric disorders including stress, sleep disturbances, and hyperarousal caused by post-traumatic stress disorder, borderline personality disorder, and other anxiety disorders.         
Clonidine along with Methylphenidate has been studied for treatment of ADHD.
An extended-release version of clonidine named Clonicel is also being studied for treatment of ADHD.
This drug may cause lightheadedness, dry mouth, dizziness, or constipation. Clonidine may also cause hypotension
Rebound hypertension on withdrawal
Clonidine suppresses sympathetic outflow resulting in lower blood pressure, but sudden discontinuation can cause rebound hypertension due to a rebound in sympathetic outflow.
Clonidine therapy should generally be gradually tapered off when discontinuing therapy to avoid rebound effects from occurring. Treatment of clonidine withdrawal hypertension depends on the severity of the condition. Reintroduction of clonidine for mild cases, alpha and beta blockers for more urgent situations. Beta blockers never should be used alone to treat clonidine withdrawal as alpha vasoconstriction would still continue.
Since ADHD drugs like amphetamine and methylphenidate tend to stimulate the sympathetic nervous system, missed doses of clonidine while under ADHD stimulant therapy might entail increased risks of a more severe rebound hypertension. This has not been evaluated.
Clonidine is a centrally-acting α-adrenergic receptor agonist with more affinity for α2 than α1. It selectively stimulates receptors in the brain that monitor catecholamine levels in the blood. These receptors close a negative feedback loop that begins with descending sympathetic nerves from the brain that control the production of catecholamines (epinephrine, also known as adrenaline, and norepinephrine) in the adrenal medulla. By fooling the brain into believing that catecholamine levels are higher than they really are, clonidine causes the brain to reduce its signals to the adrenal medulla, which in turn lowers catecholamine production and blood levels. The result is a lowered heart rate and blood pressure, with side effects of dry mouth and fatigue. If clonidine is suddenly withdrawn the sympathetic nervous system will revert to producing high levels of epinephrine and norepinephrine, higher even than before treatment, causing rebound hypertension. Rebound hypertension can be avoided by slowly withdrawing treatment.
Clonidine suppression test
Clonidine's effect on reducing circulating epinephrine by a central mechanism was used in the past as an investigatory test for pheochromocytoma, which is a catecholamine-synthesizing tumor, usually of the adrenal medulla. In a clonidine suppression test plasma catecholamines levels are measured before and 3 hours after a 0.3 µg/kg oral test dose has been given to a patient. A positive test occurs if there is no decrease in plasma levels.
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