THE PET PHARMACY By Wendy C. Brooks, DVM, DipABVP Educational Director, VeterinaryPartner.com

(for veterinary information only)

Acetylsalicylic Acid

Available in 81 mg, 325 mg & 500 mg tablets

Uses of this Medication

There are three main uses for aspirin:

• The relief of pain secondary to inflammatory disease, usually joint disease or eye disease, although safer drugs have largely replaced aspirin in this role.
• To reduce the tendency of blood to clot as in certain heart diseases, glomerular disease, and any other condition with inappropriate tendency for blood to clot.
• To reduce fever. While this is a common use for humans, we recognize that fever has an important place in immune function in fighting infection. Unless a fever is dangerously high, we generally do not reduce it artificially. Furthermore, as with the treatment of pain, safer drugs are available for this use.

How this Medication Works

Aspirin inhibits an enzyme called cyclooxygenase, which is involved in the production of inflammatory chemicals called prostaglandins. Arachidonic acid, a fatty acid that is essential in the diet of cats and dogs, makes up cell membranes. When the inflammatory cascade is active, cells begin to convert their arachidonic acid into prostaglandins. Aspirin puts a stop to this.

The problem is that there turns out to be more than one form of cyclooxygenase, and they do not all generate inflammatory prostaglandins. Some prostaglandins have important functions and we prefer not to interfere with these. Aspirin, unfortunately, does not distinguish between different forms of cyclooxygenase and inhibits them all. Because of this, in this day and age, aspirin has largely been supplanted as an anti-inflammatory drug by safer medications that primarily inhibit the form of cyclooxygenase that is associated with inflammatory prostaglandins. Aspirin is inexpensive and readily available but no longer a leading choice for pain/inflammation relief.

One prostaglandin in particular deserves special mention: thromboxane, which is involved in blood clotting. When a blood vessel is torn, specific blood cells called platelets are called to the area where they pile on top of each other to create a plug. Thromboxane is involved in keeping these platelets stuck together, constricting the blood vessel to minimize bleeding and several other more complicated operations that promote clotting. By inhibiting cyclooxygenase, thromboxane cannot be produced and the platelets present at the time aspirin is given are permanently inactivated. This is the mechanism that allows aspirin to be helpful in preventing abnormal clots, an important part of treatment for many human heart disease patients as well as pets with abnormal clotting tendencies.

There are other blood clotting mechanisms involved so aspirin does not lead to anyone bleeding to death, but surgical and dental procedures should be avoided if possible in patients taking aspirin. It is also good to know that the dose of aspirin needed to thin the blood is much lower than the anti-inflammatory dose.

Side Effects

The chief side effects of concern are:

• Nausea or diarrhea.
• Stomach and intestinal ulceration and bleeding.
• Reduced blood supply to the kidney possibly leading to kidney failure.
• Reduced ability to clot blood.

The most common side effect of aspirin is probably stomach upset. In humans this has been so common that numerous alternative pain relievers have arisen (Tylenol, Aleve, etc.) It is important to resist the temptation of using any of these non-aspirin products in pets as most have a far greater potential to ulcerate the stomach than does aspirin. As human pain relieving drugs go, aspirin is probably one of the safer ones; however, as veterinary pain relieving drugs go, aspirin is generally the last choice.

CATS METABOLIZE ASPIRIN SLOWLY AND IT IS EASY TO OVERDOSE THEM.

CHECK THE DOSE AND FREQUENCY OF ADMINISTRATION WITH YOUR VETERINARIAN.

Interactions with Other Drugs

Using aspirin with other NSAIDS (non-steroidal anti-inflammatory drugs) exaggerates the ulcerative properties of these medications. Aspirin should not be used in combination with cortisone-type medications such as prednisone, or with other NSAIDS such as Rimadylunless absolutely necessary.

Combining aspirin and furosemide leads to an increase in the aspirin’s activity. Toxicity problems could result if higher aspirin doses are used. This is unlikely to be a problem if lower (anti-clotting) doses of aspirin are being used.

Digoxin, a heart medication, will last longer and achieve higher blood levels if given with aspirin.

Spironolactone, a diuretic, will be weaker if given to a patient on concurrent aspirin.

Concerns and Cautions

It is important to understand the dosing regimen recommended if aspirin is to be used in a cat. The dosing schedule is very different for dogs than cats.

Do not use human medications on pets without specific instructions on how to safely do so from your veterinarian.

Aspirin will reduce blood flow through the kidney, which is likely to make pre-existing kidney disease much worse.

Enteric coating of aspirin may alter the way aspirin is absorbed in the stomach. The pills tend to stick to the stomach lining instead of dissolving properly. If enough tablets accumulate, overdose and death can occur. We recommend non-enteric coated aspirin only.

It is our policy not to give dosing information over the Internet.

Date Published: 1/1/2001
Date Reviewed/Revised: 07/14/2008