Isoniazid (INH)

What is isoniazid (INH)?

Isoniazid (INH) is an anti-mycobacterial drug that has been used to prevent infection of mycobacterial disease in small animals in households with humans who have been diagnosed with tuberculosis. It has also been used as part of a combination protocol to treat mycobacterial disease in cats and dogs. Treating active infections is controversial due to zoonotic transmission potential; consult with local public health officials before treating.

It is used off label (extra label) in veterinary medicine. Many drugs are commonly prescribed for off-label use in veterinary medicine. “Off label” means the drug is being used in a way that has not been reviewed or approved by the U.S. Food and Drug Administration (FDA) or Health Canada. In these instances, follow your veterinarian’s directions and cautions carefully, as they may be significantly different from those on the label.

How do I give my pet isoniazid (INH)?

• Isoniazid (INH) comes in capsule, tablet, and liquid suspension form.
• All forms are given orally (by mouth).
• This medicine is best given on an empty stomach.
• If your animal vomits or acts sick after receiving the medicine on an empty stomach, try giving the next dose with food or a small treat. If vomiting continues, contact your veterinarian.
• Measure doses carefully as this medication has a narrow therapeutic index, and there is concern for overdose and toxicity.
• Give this medication exactly as prescribed and directed by your veterinarian.

This medication should start to take effect within a few hours; however, outward effects may not be immediately noticeable.

What if I miss giving my pet the medication?

This medication must be given regularly, as directed, to be effective. If you miss a dose, give it as soon as you remember and then continue with the regular dosing schedule. If it is almost time for the next dose, contact your veterinarian for directions on dosing. Do not give the pet two doses at once.

Are there any potential side effects from isoniazid (INH)?

Isoniazid (INH) may cause the following side effects:

• Excitement
• Vomiting

Serious adverse reactions may occur. If any of the following signs develop, stop giving isoniazid (INH) and contact your veterinarian immediately:

• An allergic reaction such as irregular breathing, rash, fever, puffiness and swelling around the face
• Signs of liver toxicity (yellowing of the skin and/or mucous membranes, decreased appetite, severe vomiting, diarrhea)
• Neurological changes (e.g., changes in behavior, difficulty moving, changes in activity, unexplained pain)

Drug sensitivities can develop over time with repeated exposure. Your pet may have no reaction after the first few doses, but may develop a sensitivity later. It is important to watch for signs of an allergic reaction over the entire course of treatment.

Other side effects may occur. If you notice anything unusual, contact your veterinarian immediately. This short-acting medication should stop working within 24 hours, though effects may last longer in pets with kidney and/or liver disease.

Are there any risk factors for this medication?

DO NOT USE in:

• Pets that are allergic to any component of the medication
• Pets with acute liver disease

USE WITH CAUTION in:

• Pets with kidney disease
• Pets with decreased liver function

Are there any drug interactions I should be aware of?

The following medications should be used with caution when given with isoniazid (INH):

• Acetaminophen
• Amiodarone
• Antacids that contain calcium, magnesium, and/or aluminum
• Benzodiazepines (e.g., diazepam)
• Clopidogrel
• Corticosteroids (e.g., dexamethasone, prednisone, prednisolone)
• Other hepatotoxic drugs (e.g., azole antifungals, phenothiazines, methimazole, sulfonamides)
• Products containing cheese or fish
• Rifampin
• Theophylline
• Warfarin

Be sure to tell your veterinarian about any medications (including vitamins, supplements, or herbal therapies) that your pet is taking before starting any new treatment.

Is any monitoring needed with this medication?

• Your veterinarian may recommend baseline and periodic blood work to check liver enzyme values.
• At home, monitor for any abnormal changes. If any occur, discontinue the medication and contact your veterinarian.

How do I store isoniazid (INH)?

• Store at room temperature of 15°C to 30°C (59°F–86°F).
• Protect from light and freezing.
• Keep medication out of reach of children and pets.

What should I do in case of emergency?

If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately. If they are not available, follow their directions in contacting an emergency facility. Be prepared to provide information about the product and your pet’s weight and symptoms.