Avoiding Drug-Induced Liver Damage

This article was first printed in the February, 1998, issue of Your Dog Newsletter of Tufts University School of Veterinary Medicine.

Liver disease in dogs is serious business because of the many vital functions this important organ performs.  Most liver disease in dogs is idiopathic - that is, veterinarians can’t pinpoint the instigating cause or event.  But ironically, a known cause of canine liver ailments is adverse reaction to medication intended to help you dog.

The good news is that few of the many drugs used to treat our canine chums are known to cause liver problems. Only a handful are intrinsically harmful, causing predictable liver damage at identified doses.  Veterinarians must carefully monitor dogs receiving treatment for heartworm disease and cancer cause liver damage.  And high doses of acetaminophen are also intrinsically toxic to a dog’s liver - although cats are far more sensitive to acetaminophen than dogs.

If an adverse liver reaction to medication is going to develop, it is usually acute and occurs soon after - usually within 2 weeks of - a dog’s first dose of a particular drug.  The conspicuousness of the reaction is good news because it gives alert owners and their veterinarians the opportunity to take action before extensive liver damage occurs.

The bad news is that most adverse liver reactions are idiosyncratic - peculiar to individual dogs - and therefore unpredictable.  Veterinarians don’t know what causes idiosyncratic reactions, but they suspect either an allergy or a metabolic irregularity plays a role.  "Unfortunately, adverse drug reactions can happen at any time with almost any drug," cautions Dr. Cynthia Webster, assistant professor at Tufts University of Veterinary Medicine.

Diagnosis of drug-induced liver disease involves methods similar to those used to diagnose general liver disease - blood tests for abnormal liver-enzyme levels, imaging techniques such as ultrasound, and biopsy (extraction of tissue to detect microscopic damage to liver cells).  "Liver biopsies are very helpful diagnostic tools, and we can now perform them through the skin without surgery," notes Dr. Webster.  Finally, to implicate medication as the culprit, the owner and veterinarian must establish a time-linked relationship between drug administration and the onset of lever problems.

A fair number of dogs have livers that are idiosyncratically sensitive to anticonvulsants used to control epileptic seizures.  Phenobarbital is more liver-friendly than other anticonvulsants used in the past, but its long-term use can cause liver problems.  For this reason, some veterinarians prescribe potassium bromide for their epileptic patients. Potassium bromide coupled with lower doses of phenobarbital often allows the practitioner to effectively manage seizures.

Several other drugs have been linked to idiosyncratic liver problems.  Very high doses of anti-inflammatory corticosteroids (used occasionally to tame a dangerously overactive immune system) can harm the liver, though the low doses more commonly prescribed to treat itchy skin and other minor inflammations are not hepatically harmful.  The combination of diethylcarbamazine and oxibendazole for heartworm and hookworm prevention has caused adverse liver reactions in some dogs, as have the antibacterial combination of trimethoprim and sulfadiazine and the antifungal drug ketoconazole.  Most recently, veterinarians have discovered that carprofen, an anti-inflammatory used to treat the pain and stiffness of arthritis, causes liver anomalies in a few dogs.

As with most canine maladies, early detection of lever disease yields better prospects for recovery.  That’s why many veterinarians carefully monitor dogs taking any medications that have even a minor track record of causing liver problems.

The first and foremost treatment for a drug-induced liver malfunction is immediate withdrawal of the suspect medication.  The prognosis for recovery is good if you withdraw the drug quickly," says Dr. Webster.  A dog recovering from such an episode may also benefit from supportive care - such as intravenous fluids - even if the liver is not severely damaged.  To reduce the burden on an already compromised organ, it’s also wise to avoid giving these dogs drugs that are largely metabolized by the liver.

Drugs with advanced liver damage may require more intensive support, such as diuretics (drugs that reduce the abdominal fluid buildup that often accompanies advanced liver disease) and ammonia-inhibiting drugs and food to control hepatic encephalopathy (a brain disorder that occurs when toxic ammonia accumulates in the body due to a malfunctioning liver).

Remember that adverse liver reactions to medication are rare in the dog world.  But to reduce the risk even further, remind your veterinarian about any medications your dog is currently taking before he or she administers new medication, give all medications exactly as your veterinarian prescribes them, ask about side effects, and contact your veterinarian immediately if any signs of side effects occur.