The demands of athletic activity can take a toll on horses; specifically the joints, ligaments, tendons, bones, and muscles. Sole bruising, tendon strains, sprained ligaments and arthritic joints are common injuries in athletic horses. When these injuries occur, inflammation is the process common to all of the above. An affected area will be hot, swollen, and painful to the horse. For injuries involving the musculoskeletal system, there can be varying degrees of lameness, reflecting pain and loss of function of the affected tissues.1

Inflammation is the body’s natural response to injury. It also is the first step in the healing process. When the body’s cells are injured, they release a number of chemical substances called prostaglandins that trigger an inflammatory response. These prostaglandins increase blood flow to the damaged tissue.2

NSAID Use in Horses 

The most popular medications used in many situations in which a horse is suffering muscular pain are the non-steroidal anti-inflammatory drugs (NSAIDs). NSAIDs bind to an enzyme called cyclooxygenase (COX) that directs production of the prostaglandins. Once the drug has gained access to the inflamed area and ties up COX, there will be a decrease in the manufacture of prostaglandins and the inflammatory response will begin to subside.2

The analgesic effect of NSAIDs is also mostly due to the decrease in prostaglandin production. When present in large amounts, prostaglandins excite the nerves responsible for pain sensation. Thus, immediate pain relief following administration of an NSAID does not occur because these drugs have no effect on the prostaglandins already formed before drug administration. Fortunately, prostaglandins are short-lived in the body, and the benefits of an NSAID can usually be seen within a few hours.1

Some NSAIDs have a more direct effect on pain, working on the central nervous system to deaden pain sensation very similar to the way in which drugs like morphine control pain. In general, most NSAIDs are most effective after two to four hours, with peak effects after six to nine hours, and a decline after 12 hours.1 This explains why it is often necessary to give an NSAID twice daily to maintain anti-inflammatory effects and pain relief. However, there is considerable variation between NSAIDs when it comes to speed and the duration of anti-inflammatory effects.

Route of administration is also a consideration regarding how soon a horse will experience pain relief when NSAIDs are administered. NSAIDs generally work faster when given by IV or IM injection, although oral administration is much more convenient when the drug needs to be given for a number of days.2 

Opinions differ regarding the relative effectiveness of NSAIDs used in horses. Extensive clinical experience and the results of several comparative studies have shown that certain NSAIDs are best suited for specific problems.1 

Naproxen for Horses

Naproxen is perhaps the most effective nonsteroidal anti-inflammatory drug (NSAID) for the treatment of muscle pain and soft tissue injury in horses. Naproxen exhibits analgesic, anti-inflammatory, and antipyretic activity through its inhibition of cyclooxygenase with resultant impediment of prostaglandin synthesis.

Naproxen does not affect healing or cure underlying conditions, but it will make the horse more comfortable while recovering. Naproxen is absorbed promptly, but full clinical response may not occur for several days.

In horses, naproxen is reported to have a 50% bioavailability after oral dosing and a half-life of approximately 4 hours. Absorption does not appear to be altered by the presence of food. It may take 5 to 7 days to see a beneficial response after starting treatment. Following a dose, the drug is metabolized in the liver. It is detectable in the urine for at least 48 hours in horses after an oral dose.3

Adverse reactions to naproxen are uncommon in horses.  Gastrointestinal problems, such as ulcers, diarrhea, and GI pain may occur in some horses. Rare side effects include kidney damage, bleeding disorders, and protein loss.2 Naproxen should be avoided or very carefully monitored in horses with liver disease, kidney disease, or gastrointestinal problems.

Naproxen should not be combined with other anti-inflammatory drugs that tend to cause GI ulcers, such as corticosteroids and other NSAIDs. Naproxen may decrease the diuretic activity of furosemide.2,3

NOTE: The Association of Racing Commissioners International Uniform Classification Guidelines for Foreign Substances has designated naproxen a CLASS 4 DRUG.

Where to buy Naproxen 

Naproxen is available in the U.S. through several pharmaceutical manufacturers and through veterinary custom compounding companies. NAPROXEN 100 MG/ML by NexGen Pharmaceuticals is indicated for the treatment of a variety of musculoskeletal ailments in the horse.

Naproxen carries several potential drug interactions. Please consult your veterinarian prior to beginning any treatment regimen. 

FOR RX ONLY: A valid prescription from a licensed veterinarian is required for dispensing this medication.

1thehorse.com.

2dvm360.

3Plumb’s Veterinary Drugs.