Isoniazid Poisoning in Dogs

Isoniazid is one of the top ten medications humans take that pets become poisoned by.  People who are on them take them long term so animals have many opportunities to get into them.  While it is common in human medicine, it has a very small margin of safety in animals.  If isoniazid is prescribed to an animal, it is usually in combination with other medications to treat Mycobacterium tuberculosis.  If you believe your pet has ingested isoniazid, contact your veterinarian as quickly as possible to be able to start the detoxification process.

Isoniazid is a medication people take to treat tuberculosis (TB) or to prevent it in people who have come into contact with it.  It is also used in small animal medicine to treat atypical bacterial infections.  Liver damage is the major side effect to be concerned about.  If you believe your pet has ingested some, contact your veterinarian immediately.

Isoniazid poisoning is a serious, life-threatening condition.  Common symptoms of toxicity include:

• Vomiting
• Excessive drooling
• Diarrhea
• Tremors
• Seizures
• Abnormal heart rate
• Damage to internal organs
• Coma
• Death

Types

Isoniazid is available in pill form, liquid form or injection.  Be aware of what is in your medications and in your pet’s medications.  Since it is not an extremely commonly prescribed medication, most people know when they are taking it or giving it to their pets since it is to treat more severe illnesses.

Isoniazid is an antibacterial whose mechanism of action is to interfere with growth of tubercle bacilli, specifically the lipids and nucleic acid biosynthesis.  This is why hepatic toxicity is such a concern; since the liver is the organ that metabolizes drugs and detoxifies chemicals, it cannot perform its job correctly when overloaded with a medication like isoniazid.

You veterinarian will start with a physical exam to get a general idea of the symptoms your pet is exhibiting.  The veterinarian will also be watching for signs of neurologic toxicity and blood work will be performed to check the liver enzymes levels to see how the liver is functioning.  Blood work such as:

• Complete blood count (CBC)
• Chemistry panel
• Packed cell volume (PCV)
• Total solids
• Venous blood gases

The results will allow your veterinarian to better assess your dog’s condition and know what steps to take in the detoxification process.  Take the bottle of medication with you to the veterinarian’s office; this will allow the doctor to have an idea of what was consumed and how much.

If your dog’s blood work comes back relatively normal, it is possible your veterinarian will try to induce vomiting to prevent the toxicity from progressing.  If your pet has begun to show symptoms and their blood work is abnormal, the veterinarian may anesthetize your dog and flush your pet’s stomach to try to remove what is left of the ingested medications.   Vitamin B6 is considered the antidote in this situation and is administered as quickly as possible to combat the side effects of isoniazid toxicity.  If your dog is showing other symptoms, such as seizures, anticonvulsant medications may be administered as well.  Fluid therapy will also be started to help flush the body of the ingested isoniazid quicker.  Your pet will receive continuous observation and labs will be rechecked periodically to continue to monitor the blood chemistry levels.

Reaction time is crucial to saving the life of your pet.  If you know when or what your pet ingested, contact your veterinarian right away, they will be able to put a life saving protocol into effect immediately.  However, even if your veterinarian knows what your pet ingested, the outcome is guarded to poor.  Isoniazid does some serious damage to the liver when taken in excess; no matter how hard your veterinarian tries, if the liver fails, there is nothing more than can be done. Be sure to keep all of your medications out of reach of curious pets.  Do not store them under sinks or in low cabinets; keep them higher than your dog can reach when standing on their hind legs.  Preventing the situation from ever occurring will be easier and better for both you and your dog.