Sildenafil Exposure in a Dog

This case report was written and reviewed by members of the American Society for the Prevention of Cruelty to Animals (ASPCA) Animal Poison Control Center (APCC). The ASPCA APCC includes a full staff of veterinarians, including board-certified toxicologists, certified veterinary technicians, and veterinary assistants. The mission of the APCC is to help animals exposed to potentially hazardous substances.

History

A 1-year-old, 55 lb (24.92 kg) female golden retriever chewed open a blister pack of 100 mg Viagra (sildenafil) tablets. The dog ingested up to 3.5 tablets within a 3-hour time frame. The estimated dose of sildenafil ingested was 14.03 mg/kg. The pet arrived at the emergency clinic within 30 minutes of the latest exposure time. It is unknown if the pet was displaying any clinical signs before presentation.

Case Management

Upon presentation at the clinic, the pet was tachycardic, with a heart rate of 220 bpm (normal range, 80–120 bpm), and slightly hypertensive, with a blood pressure of 161/95 mm Hg (normal range, 160/70–160/90 mm Hg). The clinic’s veterinary team induced emesis with apomorphine (unknown route) and was able to recover some of the foil blister pack. They did not observe any evidence of the tablets.

The clinic contacted the ASPCA Animal Poison Control Center (APCC) for treatment recommendations approximately 20 minutes after the client presented the pet. The APCC advised intravenous fluid therapy (preferably lactated Ringer’s solution) and monitoring of the cardiovascular function (electrocardiogram, heart rate, and blood pressure). The clinic was advised to call for further case discussion if needed.

The owner called the next day around 12:22 CST with an update. It was noted that the pet recovered and was discharged from the clinic approximately 4.5 hours after presentation. The dog did not require any additional medical treatment beyond monitoring and fluids.

Discussion

Sildenafil: Pharmacokinetics and Pharmacodynamics

Sildenafil is a medication that is used for erectile dysfunction in humans. It is classified as a vasodilator as well as a phosphodiesterase type V inhibitor.¹ The 2 other medications that are used for erectile dysfunction (vardenafil and tadalafil) act similarly.

Sildenafil affects cyclic guanosine monophosphate (cGMP), which is found in areas such as the corpus cavernosum and pulmonary vasculature. The inhibition of phosphodiesterase V leads to an increase in cGMP; this causes smooth muscle relaxation, which then leads to a decrease in blood pressure (hypotension).¹ If the patient suffers from preexisting hypotension or is taking medications that can decrease blood pressure, sildenafil can intensify the effects.

Sildenafil is increasingly being used in veterinary medicine for dogs that have pulmonary hypertension. When used therapeutically, the dose can range from 0.5 to 3 mg/kg orally, every 8 to 12 hours.^1,2 Sildenafil is often more cost-effective than other medications used for this purpose, making this the more ideal medication for veterinary patients.³

The onset of action for sildenafil is 30 minutes to 2 hours after the exposure. The primary risks are hypotension and tachycardia. However, lesser clinical signs can include lethargy, ataxia, and behavior changes.

Diagnosis and Treatment of Sildenafil Exposure

Diagnosing a sildenafil exposure can be difficult because many other health concerns or toxins can affect the heart rate and blood pressure. Diagnosis of sildenafil toxicosis is based on history of an exposure and clinical signs. Rule-outs should include other hypotensive agents or conditions, such as calcium channel blockers, angiotensin-converting enzyme inhibitors, beta blockers, cardiac disease, shock, or trauma.

Diagnosis can be determined by confirming with the owner that the medication is in the home and that the pet could have had access to it. Peak plasma levels occur within 30 to 120 minutes, so identifying a time frame of possible exposure is important as well. There are no current blood tests available to determine the presence of sildenafil for the purpose of treatment.

If the exposure was within 30 minutes, emesis may be induced as long as the pet is asymptomatic and no other contraindications exist. If the animal is symptomatic, decontamination with emesis is not recommended because the medication is already being absorbed into the system. With symptomatic patients, treatment depends on the clinical signs the pet is displaying. Hypotension and tachycardia generally respond well to IV fluid therapy, which helps reestablish the blood pressure. If fluids are not sufficient to support the blood pressure, vasopressors such as dopamine (1–3 mcg/kg/min constant-rate infusion¹) can be included.

Conclusion and Clinical Relevance

Sildenafil can be used to treat erectile dysfunction in humans and pulmonary hypertension in pets and humans. Questioning the owner about medications in the home, even if they do not think there could have been an exposure, is important while obtaining the patient’s history. Most symptomatic patients have resolution of clinical signs within 24 to 48 hours after ingestion. Depending on the patient’s signalment and the time frame in which treatment is started, some patients are able to recover more quickly. No long-term effects are expected.