Clonidine

Clonidine is a direct-acting α2 adrenergic agonist prescribed historically as an antihypertensive agent. It has found new uses, including treatment of some types of neuropathic pain, opioid detoxification, sleep hyperhydrosis, 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), where it's given in late afternoon and/or evening for sleep, and because it sometimes helps moderate ADHD-associated impulsive and oppositional behavior, and may reduce tics.^[1] Clonidine can also be used in the treatment of Tourette syndrome.^[2]

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Indications

The main use for this medication is used to treat high blood pressure. It works by stimulating certain brain receptors (alpha adrenergic type) which results in the relaxing of blood vessels in other parts of the body, causing them to widen. It has specificity towards the presynaptic alpha 2 receptors in the Vasomotor Center in the CNS. This binding inhibits the production of NE (Norepinephrine), thus decreasing sympathetic Outflow. The parasympathetic activity predominates.

 
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.

Non-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, attention deficit hyperactivity disorder.^[3][4]

Clonidine is regularly prescribed to opiate addicts to help alleviate their withdrawal symptomology. It is mainly used to combat the sympathetic nervous system response to opiate withdrawal, namely tachycardia and hypertension, in the first couple days of withdrawals.^{[citation needed]} It helps take away the sweating, hot/cold flashes, and general restlessness. The sedation effect is also useful.^{[citation needed]}

Adverse effects

This drug may cause drowsiness, lightheadedness, dry mouth, dizziness, or constipation. Clonidine may also cause hypotension.^[5] Sucking on sugarless hard candy or ice chips, chewing sugarless gum, drinking water, and using a saliva substitute may help relieve dry mouth.

Pharmacodynamics

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 pheochromocytomae, which are catecholamine-synthesizing tumors, 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.