Minoxidil toxicosis in felines

Many active ingredients of human medications applied to the skin, such as calcipotriene/calcipotriol, fluorouracil, flurbiprofen and retinoid, are highly toxic to pets – particularly to cats – and this at very low doses.

Minoxidil is an over-the-counter topical product (5% foam, 2% and 5% solution) used in human medicine for the treatment of androgenetic alopecia (AGA) that affects both men (up to 50%) and women, due to an excessive response to androgens.

Minoxidil acts as an anti-androgen primarily by inhibiting the 5α-reductase enzyme that reduces the conversion of testosterone to the dihydrotestosterone (DHT). DHT is an endogenous androgen involved in the growth and repair of hair follicles.

Minoxidil was first introduced in the 1970s as an oral treatment for severe refractory hypertension in humans, owing to its potent vasodilator properties. Studies demonstrated the efficacy of low-dose oral minoxidil for AGA in 1986. Hypertrichosis, a side effect, has led to the development of topical formulations for the treatment of AGA.

What is the toxic dose and mechanism of toxicity of minoxidil?

Minoxidil is a piperidine pyrimidine derivative, a small molecule (209 daltons) with logP value of 1.24. Despite a half-life of approximately four hours, the duration of action of a single dose may be at least 48 hours.

The number of reported cases of accidental exposure toxicosis to topical minoxidil in pets has increased sharply in recent years in parallel with increased usage of minoxidil for the treatment of hair loss. Very small amounts of minoxidil can be toxic to cats and dogs, and even a single lick by a cat could be deadly.

Routes of minoxidil exposure include oral ingestion, contact with fur or skin, or both. Reported exposures in pets include contact with spilled solution or an open bottle; chewing discarded bottles; licking a person’s freshly treated scalp, beard or hands; and licking human bedding or pillows contaminated with residual topical minoxidil. Cats are more sensitive than dogs, with a higher case fatality rate.

Minoxidil is primarily metabolised in the liver to minoxidil glucuronide and minoxidil sulfate, the active form (14 times more potent than minoxidil). Cats have an increased sensitivity to this drug compared to dogs due to differences in the metabolism, including:

• Reduced hepatic capacity for key metabolic process such as glucuronidation (UDP-glucuronosyl transferase enzymes deficiency).
• Limited capacity for sulfate conjugation, the major pathway of pharmacokinetics in cats (sulfotransferase enzyme SULT2A1 deficiency).

Minoxidil exerts toxic effects through multiple mechanisms.

Potassium channel opener

Minoxidil binds to intracellular adenosine triphosphate-sensitive potassium channels expressed in the arteriolar vascular smooth muscle, heart, pancreas and brain. When a potassium ion (K⁺) channel opens, K⁺ efflux increases, causing lowered membrane potential (hyperpolarisation of the cells), closure of voltage-activated calcium (Ca²⁺) channels, decreased Ca²⁺ entry, resulting in the relaxation of smooth muscle within the arterioles. Vasodilation may persist for up to 72 hours, and severe hypotension can lead to circulatory shock. Hypotension also causes cells to switch to anaerobic metabolism, producing excess lactate and resulting in metabolic acidosis.

Arteriolar vasodilation caused by minoxidil stimulates the sympathetic nervous system via carotid and aortic baroreceptor reflexes, resulting in tachycardia, increases cardiac output (volume of blood pumped by the heart per minute) and myocardial contractility.

Opening potassium channels in the brain decreases neuronal excitability, leading to lethargy. Minoxidil activates the renin-angiotensin system in the kidneys, resulting in an increase of biosynthesis of aldosterone and subsequent sodium and water retention. It also inhibits the release of insulin on pancreatic β-cells, resulting in hyperglycaemia.

Cardiac toxicity

Severe tachycardia and high cardiac output lead to acute imbalance between myocardial oxygen demand and supply, resulting in myocardial infarction.

Pulmonary oedema

Minoxidil has a potent dilator action on the pulmonary vascular smooth muscle. Relaxation of smooth muscle within the pulmonary arterioles leads to decreased pulmonary vascular resistance and fluid accumulation in the lungs, leading to pulmonary oedema.

What are the clinical features of minoxidil  poisoning in pets?

Clinical effects have a rapid onset, usually occurring within 30 minutes after ingestion (rapid absorption and peak plasma level in first hour). They include the following:

• GI signs
  • Anorexia.
  • Vomiting.
  • Excessive drooling.
• Neurological signs
  • Depression.
  • Acute lethargy.
• Cardiovascular signs
  • Severe hypotension (may persist for more than 24 hours).
  • Prolonged high tachycardia (more than 180bpm in cats, 200bpm to 220bpm in dogs).
  • Pericarditis
  • Peripheral oedema (sodium retention).
• Respiratory signs
  • Cough.
  • Dyspnoea.
  • Tachypnoea.
  • Pleural effusion (dullness to percussion, diminished breath sounds).

Death comes from cardiac failure.

Laboratory values indicate:

• Azotaemia. Blood urea nitrogen (reference range 3.5mmol/L to 8.0mmol/L in cats; 2.5mmol/L to 9.6mmol/L in dogs).
• Elevated cardiac troponin I concentrations (reference range: 0ng/ml to 0.09ng/ml in cats; 0.03ng/ml to 0.07ng/ml in dogs).
• Hyperglycaemia (reference range: 3.4 mmol/L to 5.7mmol/L in cats; 5.6mmol/L to 13.9mmol/L) in dogs.
• Hypokalaemia (reference range: 3.42mmol/L to 4.71mmol/L in cats; 3.5mmol/L to 5.1mmol/L in dogs).
• Hypocalcaemia (normal values for ionised calcium [iCa] in cats: 1.12mmol/L to 1.32mmol/L; in adult dogs: 1.25mmol/L to 1.45mmol/L; young dogs up to two years of age have serum iCa concentrations that are higher than reported in older animals). iCa is more stable in serum than in whole or heparinised blood.
• Metabolic acidosis (lactate levels reference range: 0.5mEq/L to 2.5mEq/L in cats; 0.3mEq/L to 2.5mEq/L; standardised base excess less than -5mmol/L in cats and dogs).
• ECG shows ST segment depression and T-wave inversion reflecting myocardial infarction.

How should minoxidil poisoning in cats be managed?

Management of minoxidil toxicosis in pets is a life-threatening emergency, necessitating immediate veterinary intervention. Treatment primarily involves intense supportive care.

Supportive therapy

Treating respiratory distress

Attention to airway and breathing is paramount. Oxygen saturation of less than 93% and an arterial partial pressure of oxygen less than 70mmHg should prompt oxygen supplementation. Affected animals should be intubated with a cuffed endotracheal tube to keep the airway clear and provided oxygenation.

The first treatment is immediate oxygen supplementation by high-flow oxygen therapy (HFOT). HFOT is the administration of warm humidified oxygen via nasal prongs. HFOT is an effective method of delivering high volumes and pressures of oxygen.

HFOT bridges the gap to mechanical ventilation, is less invasive than ventilation, and is generally more cost effective. It allows the delivery of higher flow rates of oxygen (4L/min to 60L/min with some devices). The flow rate is set to meet or exceed the inspiratory flow demand of the patient.

In normal mesocephalic dogs (for example, the golden retriever, Labrador retriever and German shepherd dog), the average flow demand is approximately 500ml/seconds to 1,000ml/seconds. Brachycephalic dogs (for example, the boxer and bulldog) may have lower flow demands.

Also, administer glucocorticoids: dexamethasone sodium 0.15 mg/kg to 1mg/kg IM or IV single dose every 24 hours. Alternatively, administer methylprednisolone sodium succinate 20mg/kg to 30mg/kg IV, which may be repeated 4 to 6 hours for 24 to 48 hours.

Treat pulmonary oedema

Administer loop diuretic: furosemide 2mg/kg to 4mg/kg IV. These large doses may need to be repeated (initially every one to two hours) until the respiratory rate and dyspnoea start to decline. Once stable, the dose should be reduced to 0.5mg/kg to 2mg/kg every 8 to 12 hours, as dictated by clinical status.

Treating hypotension

Fluid therapy is ineffective in the treatment of profound hypotension related to minoxidil toxicosis. Administer the vasopressor of first choice: norepinephrine 0.05µg/kg/min to 1µg/kg/min CRI. These drugs should be used cautiously.

Blood pressure should be monitored every 15 minutes for the first hour in emergent situations, then hourly thereafter if the patient is critical, or every six hours once the patient is stable. Norepinephrine is preferred to dopamine, which may worsen tachycardia.

Treating tachycardia

Administer esmolol, a beta blocker: loading dose of 200µg/kg to 500µg/kg IV over 1 minute, followed by a 25µg/kg/min to 200µg/kg/min CRI.

Treating metabolic acidosis

Administer sodium bicarbonate 8.4% solution at 2ml/kg to 5ml/kg bodyweight CRI, depending on the severity of the acidosis, over a four-hour period.

Treating vomiting

Severe vomiting should be treated by antiemetic serotonin antagonists such as ondansetron: 0.1mg/kg to 0.2mg/kg IV slow bolus every 8 to 12 hours in dogs, 0.5mg/kg IV loading dose followed by 0.5mg/kg/hour CRI for 6 hours in cats; or dolasentron: 1mg/kg IV slow bolus every 24 hours in dogs and cats.

Alternatively, administer maropitant (neurokinin-1 antagonist) 1mg/kg IV, once a day, for up to five consecutive days.

Detoxification therapy

If dermal exposure has occurred within the past 24 hours, wash the pets thoroughly with warm water and a mild detergent.

The induction of emesis is contraindicated. Gastric decontamination or activated charcoal cannot be given because of its high absorption rate. Treatment of lipophilic drug toxicosis such as minoxidil (logP value greater than 1.0) by intravenous lipid emulsion (ILE) should be considered (“lipid sink”). Administer ILE 20% IV bolus 1.5ml/kg over one minute, followed by a CRI of 0.25ml/kg/min for the next 30 to 60 minutes.

In non-responsive patients, an additional intermittent bolus can be given IV slowly at up to 7ml/kg. If clinical signs do not improve after 24 hours, discontinue ILE. ILE 20% preparations are isotonic and can be given by a peripheral vein or in a central catheter using aseptic techniques. The patient should be observed for a minimum of six days.

To keep pets safe from minoxidil exposure, follow these precautions:

• Keep minoxidil products completely out of reach of children and pets.
• Wash your hands immediately after every application.
• Cotton swabs, tissues, or other materials that have absorbed minoxidil should be discarded in a sealed trash can inaccessible to pets.
• Keep cats away from treated areas (for example, do not let cats lick your skin or sleep on your pillow after use of minoxidil).
• Educate family and friends about minoxidil toxicosis in cats and dogs.
• Manufacturers should consider warning labels highlighting the risks of topical minoxidil to pets.
• The use of some of the drugs in this article is under the veterinary medicine cascade.
• This article appeared in Vet Times (11 August 2026), Volume 56, Issue 32, Pages 14-16

Lotfi El Bahri qualified as a doctor in veterinary medicine in Toulouse in 1969. In 1976, he joined the National Veterinary School at Sidi-Thabet in Tunisia. He gained a master in veterinary science in 1977 and, in 1981, a PhD in Alfort, France. Lotfi is a professor of pharmacology and toxicology, an analyst and a pharmacotoxicology expert in veterinary drugs.