Clonidine Toxicity

Updated: Oct 21, 2021
Author: David Riley, MD, MSc, RDMS, RDCS, RVT, RMSK; Chief Editor: Michael A Miller, MD 


Practice Essentials

Clonidine is a central alpha-agonist that is used as an antihypertensive agent. Other reported clinical uses include treatment of opiate and alcohol withdrawal[1] and control of atrial fibrillation with a rapid ventricular rate. It is also used for the following:

  • Pediatric preanesthesia
  • Pediatric postoperative pain
  • Attention-deficit/hyperactivity disorder (ADHD)
  • Migraine headaches
  • Nicotine addiction
  • Menopausal flushing
  • Attention deficit disorder [2]
  • Tourette syndrome [3]
  • Pediatric panic and anxiety disorders

At therapeutic doses (0.2-0.9 mg/d), clonidine is commonly associated with adverse effects such as dry mouth, sedation, dizziness, and constipation. While generally safe, at toxic doses clonidine can cause serious cardiopulmonary instability and central nervous system (CNS) depression in children and adults.

Clonidine is available in a weekly transdermal patch (Catapres TTS: 0.1 mg, 0.2 mg, or 0.3 mg/d, with each patch containing 2.5 mg, 5 mg, and 7.5 mg of clonidine, respectively) and in tablet form (Catapres: 0.1 mg, 0.2 mg, and 0.3 mg; Combipres includes 15 mg of chlorthalidone diuretic).

Clonidine can also be compounded as a liquid formula.[4]  An ophthalmic solution is occasionally used in the treatment of glaucoma.



Clonidine is an imidazole derivative and was first used as a nasal decongestant. Decongestants containing tetrahydrozoline, also an imidazole derivative, can result in the same signs and symptoms as clonidine poisoning when ingested, especially in children.

Clonidine acts primarily as a presynaptic CNS alpha2-agonist, stimulating receptors in the nucleus tractus solitarii of the medulla oblongata. This inhibits sympathetic outflow, which results primarily in a reduction of sympathetically mediated vasoconstriction, cardiac inotropy, and chronotropy.

Clonidine also has peripheral alpha1-agonist activity, which may produce transient vasoconstriction and hypertension early in overdose when peripheral drug levels may be transiently higher than levels in the CNS.

Clonidine is rapidly absorbed from the gastrointestinal tract and has excellent CNS penetration because of lipid solubility. Peak plasma concentrations are reached 3-5 hours after a single oral dose. Dermal application may take several days to reach steady state levels. No known pharmacologically active metabolites exist. Plasma half-life is 12-16 hours, with the antihypertensive effects occurring within 30-60 minutes of ingestion. Clonidine is excreted unchanged in the urine and is metabolized by the liver.


In the 2019 Annual Report of the American Association of Poison Control Centers' National Poison Data System, 5596 single exposures to clonidine were reported. Of those, 3503 were unintentional toxicities. In 2019, 1724 were in patients younger than 6 years, 2341 were in patients 6-19 years old, and 1442 were in patients 20 years of age and older.[5]


Prognosis is generally good for patients who present early and have had prompt and proper treatment. Mortality is rare with a small number of reported deaths. Morbidity, in terms of cardiorespiratory and CNS dysfunction, generally tends to be more severe in young persons than in adults.

Of the 5596 reported toxic exposures to clonidine in 2019, 3987 were treated in a health care facility. Of this subset of patients, 1039 had no significant outcome, 1032 had minor effects, 1889 had moderate morbidity, and 233 had major morbidity and 2 deaths were reported.[5]

Patient Education

Children may be easily affected by relatively small doses of clonidine. Educating patients about the importance of keeping clonidine and all drugs out of children's reach is critical.

For patient education information, see First Aid for Poisoning in Children and Child Safety Proofing.




While elucidating the amount and timing of the clonidine ingestion is helpful, in practice, signs and symptoms guide therapy. Always suspect other co-ingestants and screen appropriately.

Children are particularly susceptible to toxic reaction from small doses (ie, normal adult therapeutic doses) of clonidine.

The Catapres TTS patch appears similar to a small Band-Aid or sticker, and a child could pull the patch off a sleeping caretaker. Several case reports document patches detaching spontaneously from a sleeping parent in a bed shared with a child and subsequently adhering to the child with resultant toxicity. In cases of possible clonidine toxicity involving children, always question family, friends, and emergency medical services (EMS) as to whether a child may have had access to clonidine.

Irritability may be noted.

Three patients who were receiving long-term treatment with intrathecal clonidine experienced a clonidine overdose because of inadvertent extravasation during the refilling procedure. All three patients experienced loss of consciousness and severe systemic hypertension that required aggressive parenteral treatment. Inadvertent injection of clonidine into the subcutaneous pocket rather than into the reservoir is rare, but it is very dangerous because the drug cannot be retrieved and massive doses are involved.[6]

Physical Examination

Findings on physical examination may include the following:

  • Serious respiratory depression and apnea may be present.
  • Bradycardia has been reported. [7]
  • Hypotension is very common; however, hypertension (usually transient) may occur initially.




Approach Considerations

Clonidine levels have not been shown to correlate with toxicity and should not be routinely drawn. Measure electrolyte and glucose levels to screen for anion gap acidosis or hypoglycemia.

Serum toxicology screen may be useful if the patient is not responding to standard measures; has significant symptoms and a co-ingestion is suspected; or is comatose. It may also be useful if a specific drug level is desired. Acetaminophen levels should be obtained if overdose is suspected.

Other studies to consider include the following:

  • Test for pregnancy in women of childbearing age.
  • Perform a 12-lead electrocardiogram (ECG) in addition to continuous cardiac monitoring.
  • Obtain a noncontrast cranial computed tomography (CT) scan if hemorrhagic bleeding, stroke, or head trauma is suspected.
  • Consider an emergent magnetic resonance imaging (MRI) scan if concern exists over a brainstem infarct or a change in management may result.
  • Lumbar puncture may be indicated in patients with altered mental status and suspicion of CNS infection.


Prehospital Care

Provide aggressive supportive care because patients may rapidly decompensate. Address airway, breathing, and circulation (ABCs) as usual. Intravenous access with crystalloid and pressor support with dopamine may be necessary.

If a clonidine patch is present on the skin, remove it and wash the exposed area. Initiate standard naloxone therapy and blood glucose checks.

Continuous electrocardiographic (ECG) monitoring should carry over to the emergency department (ED).

Prehospital ipecac syrup administration is contraindicated.

Emergency Department Care

Focus initial treatment on the ABCs. Clonidine toxicity can cause serious respiratory depression and apnea requiring immediate endotracheal intubation and mechanical ventilation. Once the airway is secure, place the patient on continuous ECG, blood pressure, and oxygen saturation monitoring. Place at least one large-bore IV line. Consider central venous pressure (CVP) monitoring in patients who are markedly hypotensive.

Clonidine toxicity can cause serious respiratory depression and apnea requiring immediate endotracheal intubation and mechanical ventilation.

Hypotension is very common with clonidine toxicity; initially treat the patient with aggressive crystalloid infusion. If aggressive volume resuscitation fails to raise blood pressure, consider pressors such as dopamine and epinephrine. Maintain good urine output because clonidine is excreted at least 50% unchanged in the urine.

Bradycardia, either sinoatrial (SA) nodal or AV nodal, has been reported with clonidine toxicity.[7] Atropine is the first-line drug of choice. Consider dopamine if atropine fails with SA nodal, first-degree, or Mobitz I AV nodal block; however, in Mobitz II and third-degree AV nodal block, atropine is only temporizing until definitive pacing is initiated.

Transcutaneous pacing is quicker to initiate, but causes the patient more discomfort than transvenous pacing. Consider transvenous pacing in patients with massive ingestions who have third-degree AV nodal block.

Hypertension may occur initially from peripheral alpha1-agonist activity and vasoconstriction. This hypertension is usually transient and does not require treatment; if hypertension is severe, symptomatic, and prolonged, treatment with a short-acting agent such as intravenous nitroprusside can be considered.

Administer activated charcoal by mouth or nasogastric tube for clonidine toxicity in a 1-g/kg dose (standard for toxic ingestions). If significant CNS depression exists, intubate before administering activated charcoal to prevent aspiration. The clinician should be aware that even intubated patients are at risk of activated charcoal aspiration. Lavage is controversial; yet consider it if ingestion is significant and occurred less than an hour before arrival.

Naloxone (Narcan) may treat clonidine toxicity. It improves the mental status of adults and children who have ingested toxic amounts of clonidine; this, however, has not been universal and naloxone can cause hypotensive and hypertensive responses.[8] Naloxone can also has been reported to cause severe hypertension.[9]

The American Academy of Pediatrics[10] recommends a dose of naloxone of 0.1 mg/kg for infants and children up to age 5 years or weighing 20 kg. Children older than 5 years or weighing more than 20 kg may be given 2 mg of naloxone. Adults with isolated clonidine toxicity may be given 2 mg doses of naloxone, titrated to effect. The clinician should be aware that naloxone administration in chronic opioid users can precipitate withdrawal with consequent vomiting and risk of aspiration.

Provide symptomatic and supportive care, the main therapy for clonidine toxicity. Passively warm patients who have hypothermia. Most comas resolve with supportive measures.

Two case reports document yohimbine reversal of clonidine toxic states.[11] Yohimbine is a central alpha2-adrenergic antagonist with effects that directly oppose clonidine, making it a theoretically useful antidotal agent.[11]

Admit significantly symptomatic patients with clonidine toxicity to the intensive care unit (ICU). A ward admission on a monitor is probably reasonable for minimal symptoms if the patient has been observed for several hours with improvement or without worsening. Remember that patients demonstrating clonidine toxicity, secondary to transdermal exposure, may experience a prolonged period of symptoms from a prolonged half-life secondary to a "depot" effect in the subdermal tissues.

Transfer patients with clonidine toxicity if the potential benefits outweigh the risks.

Patients with suspected clonidine ingestion may be discharged if they remain asymptomatic for 4-6 hours and have normal vital signs. Obtain a psychiatric evaluation before discharge for patients with suspected intentional ingestion.


Unless the treating physician has extensive experience with acute poisonings or if significant toxicity manifests, contacting a poison control center for advice and feedback is reasonable. A formal toxicology team may provide valuable input.

Medical Care

In general, treat with supportive care. Administer activated charcoal to all patients at risk for aspiration after intubation. Temporary support for symptomatic cardiovascular effects of clonidine also may be necessary.



Cardiovascular Agents

Dopamine (Intropin)

DOC in patients with clonidine toxicity who remain hypotensive after IVF. Stimulates adrenergic and dopaminergic receptors. Hemodynamic effect is dependent on dose. Lower doses predominantly stimulate dopaminergic receptors that, in turn, produce renal and mesenteric vasodilation. Higher doses produce cardiac stimulation and renal vasodilation.

Atropine IV/IM (Atropair)

Vagolytic for patients with bradycardia (eg, SA or AV nodal block).

Doses < 0.5 mg can produce a paradoxical reaction.


Naloxone (Narcan)

Absolute benefits are unproven and rates of success vary. Consider a drip if significant improvement results.

GI Decontaminant

Activated charcoal (Liqui-Char)

Emergency treatment in poisoning caused by drugs and chemicals. Network of pores present in activated charcoal adsorbs 100-1000 mg of drug per gram of charcoal. Does not dissolve in water.

For maximum effect, administer within 30 min of ingesting poison.


Questions & Answers