Naproxen toxicity typically results from administration of an improper dose by well meaning owners or acute overdose from curious animals eating large quantities.

Naproxen belongs to a class of drugs known as non-steroidal anti-inflammatory drugs (NSAIDs). The purpose of these drugs is to reduce the pain associated with inflammation. NSAIDs work by inhibiting the production of prostaglandins. There are different types of prostaglandins and, as a class, are responsible for a wide variety of normal body functions. They are present in areas of trauma to help repair damage. They also maintain proper blood flow to the kidneys and protect the stomach lining from the effects of stomach acids.

The primary reason NSAIDs are used is to reduce the presence of prostaglandins in trauma related injuries. Without prostaglandins, there is less inflammation and therefore, less pain. Unfortunately, medicine has yet to make an NSAID that only affects the prostaglandins associated with inflammation. When the prostaglandins for inflammation are inhibited, so are those responsible for normal kidney blood flow and stomach protection. This is the cause of the toxic properties of naproxen in pets.

The toxic signs of naproxen include:

Vomiting

Depression

Anorexia

Diarrhea

Stumbling

Dogs are particularly sensitive to the stomach ulcer effect of naproxen. Stomach ulcers can occur within 12 hours of ingestion but sometimes can take up to four days. Cats are more sensitive to the kidney effects of naproxen. With massive overdose or ingestion, severe kidney impairment can occur within 12 hours of ingestion but may take up to five days. In severe cases, seizures may occur.

Naproxen can result in stomach ulcers at a dose of 2.5 mg per pound (6 mg/kg) daily. Naproxen can result in kidney failure at a dose of 7 gm per pound (15 mg/kg).