Pyrimethamine + Sulfadiazine

What is pyrimethamine + sulfadiazine?

Pyrimethamine + sulfadiazine (ReBalance®) is a drug used for the treatment of equine protozoal myeloencephalitis (EPM) caused by Sarcocystis neurona in horses.

While pyrimethamine + sulfadiazine is labeled for use in horses, it may be used off label to treat neosporosis in dogs or toxoplasmosis in cats. 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 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 pyrimethamine + sulfadiazine?

• Pyrimethamine + sulfadiazine is given orally (by mouth) and comes in the form of an oral suspension.
• It is recommended to give pyrimethamine + sulfadiazine on an empty stomach, one hour before feeding.
• Follow your veterinarian’s directions and measure liquid forms of this medication carefully.
• Shake well before using.
• Do not handle this medication if you are pregnant.

This medication may take several weeks before it fully takes effect. Improvement in clinical signs will be noted gradually.

What if I miss giving my pet the medication?

If you miss a dose, give it as soon as you remember, and then continue with the regular dosing schedule. However, if it is almost time for the next dose, skip the missed dose and resume the regular dosing schedule. Do not give the pet two doses at once.

Are there any potential side effects from pyrimethamine + sulfadiazine?

Pyrimethamine + sulfadiazine may cause the following side effects:

• Diarrhea or loose stools
• Reduced appetite
• Itching
• Mild colic in horses
• Worsening neurological signs associated with EPM, such as weakness or trouble walking, that may last up to 5 weeks (these signs may be due to inflammation caused by dying parasites in the central nervous system tissue)

Serious adverse reactions associated with this drug may occur without warning and could, in rare cases, lead to death. If any of the following signs develop, stop giving pyrimethamine + sulfadiazine and contact your veterinarian immediately:

• Extreme tiredness or lack of energy
• Bleeding or bruising
• Pale gums
• Loss of appetite
• Fever
• Seizures

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 the treatment and notify your veterinarian if you have any concerns.

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

Are there any risk factors for this medication?

DO NOT USE pyrimethamine + sulfadiazine in:

• Pets that are allergic to pyrimethamine, sulfadiazine, or other sulfonamide drugs
• Pets that will be used for human consumption

USE WITH CAUTION in:

• Pets with liver or kidney problems
• Pets with bone marrow problems or other blood disorders
• Pets that are being treated with other drugs that can affect bone marrow
• Pets that are pregnant or nursing

Are there any drug interactions I should be aware of?

The following drugs should be used WITH CAUTION when given with pyrimethamine + sulfadiazine:

• Antacids
• Azothioprine
• Cyclosprorin
• Methotrexate, phenylbutazone, thiazide diuretics, salicylates, probenecid, phenytoin, and other highly protein-bound drugs
• Other sulfonamides, such as methanomine
• Para-aminobenzoic acid (PABA)
• Trimethoprim
• 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?

• A complete blood count should be done to assess the number and health of red cells, white cells, and platelets before starting this medication and at least monthly during the treatment period.
• At home, monitor for any side effects such as those noted above. If any occur, stop using the medication and contact your veterinarian.
• Your veterinarian will let you know when and how they want to reassess your pet to ensure the medication is working and not causing side effects.

How do I store pyrimethamine + sulfadiazine?

• Store this medication in its original container in a cool, dry place and protected from light.
• Store this medication at room temperature between 68°F and 77°F (20°C and 25°C).
• Store compounded forms of this medication according to the label.
• Protect from freezing.
• Keep out of reach from pets and children.

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.