Researchers in Colorado are investigating whether acetaminophen—a unique anti-inflammatory medication—can help manage uveitis and other painful equine eye conditions.

“There are actually several groups exploring potential uses of acetaminophen in horses,” says Kathryn L. Wotman, DVM, of Colorado State University. “Acetaminophen itself isn’t new or unusual—it’s the active ingredient in the Tylenol you probably have in your own medicine cabinet—but it’s not used very much in equine veterinary medicine.”

An alternative to traditional NSAIDs

The drug, however, may prove to be a viable alternative to traditional NSAIDs, like phenylbutazone (bute) and flunixin meglumine (Banamine).

“It works on different pain pathways than other drugs, so it could potentially work better for different types of pain,” says Wotman. “It also isn’t associated with some of the side effects we see with other NSAIDs, so it might be a good alternative for horses who can’t have bute or Banamine. It’s also inexpensive compared to other NSAIDs, so that’s good.”

Difficult to manage ocular conditions

The Colorado study focused on the treatment of ocular conditions because they can be difficult to manage. “Part of the problem is eyes are very painful to treat,” says Wotman. “And it is difficult to get drops into a horse’s eye. Horse owners typically have to use ointments or have a treatment system placed (subpalpebral lavage), and it’s still tricky.”

Another obstacle is many systemic drugs fail to reach the aqueous humor, the clear fluid that fills the front part of the eye. “There is a blood/ocular barrier, just like there’s a blood/brain barrier,” explains Wotman, “and unless a drug can penetrate that, it won’t have much anti-inflammatory or analgesic effect on the eye.”

For the experiment, the Colorado team selected six healthy horses with no history of eye problems. The horses were given a 20 mg/kg dose of acetaminophen every 12 hours orally for a total of six doses. The researchers then took blood and aqueous humor samples from the horses to test for concentrations of the drug.

Promising results

The results were promising: Acetaminophen was detected in both serum and aqueous humor of all the study horses. “This means the drug is reaching the area,” says Wotman. “Which is a good first step. The next step is to determine if it’s having the desired effect.”

The Colorado team is mid-way through a clinical trial to determine if acetaminophen can relieve pain associated with uveitis, ulceration and other painful ocular problems in horses.

“If it is effective, that could be a great help to these horses,” says Wotman. “[Acetaminophen] is somewhat synergistic with other NSAIDS, meaning we might be able to alternate it with something like Bana-mine for even greater pain relief. For horses dealing with eye pain, that would be a terrific option.”

Reference: “Ocular penetration of oral acetaminophen in horses,” Equine Veterinary Journal, December 2022