Methocarbamol, frequently known by its trade name of Robaxin, is a widely used therapeutic medication in horses. It’s also one of the medications we have recently seen the most overages for.
Most people think of methocarbamol as a muscle relaxant that’s used to manage horses who tie up after exercise, or who are struggling with muscle spasms or muscle pain related to an injury. It’s frequently prescribed for back soreness.
You may have heard some buzz about methocarbamol recently, and there are a few reasons for that. For one thing, we’ve seen more positive drug tests for it than normal. Methocarbamol is a quantitatively restricted drug, meaning it’s permitted to be in the horse up to a certain concentration at the time of testing. This concentration reflects an administration of a recommended standard dose no closer than 12 hours prior to competition. With all quantitatively restricted drugs, that timeframe is important; giving a medication a few hours after the planned time can result in a positive test, which is why we recommend having one person in each barn responsible for administering medications on an organized, preset schedule. It also helps to think about how the medication is being given to the horse.
“We’ve had people tell us recently that they give a quantitatively regulated drug by putting it in the horse’s feed,” said Dr. Stephen Schumacher, Chief Veterinary Officer at US Equestrian. “Well, if you have a horse who doesn’t finish their evening feed until the middle of the night, they’re getting methocarbamol within the 12-hour window. That’s something we want you to be aware of.”
You also need to be aware of the dosage you’re administering. Some oral medications, including methocarbamol, are available in the same form (such as a tablet) but in multiple concentrations. If you buy a new bottle from a new prescription, make sure you’re dosing correctly based on the concentration you have.
Injectable methocarbamol is no longer available in a mass-manufactured, FDA-approved form, which means the injectable versions in use today are compounded. Compounding pharmacies serve an important role in situations like these where a medication is temporarily or permanently unavailable, but it’s important to know that there can be some risk with compounded products. There can be significant differences in the internal controls at compounding pharmacies, and because they’re required to make medications in small batches as prescribed, there can be differences in concentrations from one batch to another. Increasingly, compounding pharmacies are marketing straight to consumers even though their products require a prescription to order. Always work with your veterinarian to find a compounding pharmacy with good quality controls and a track record of staying compliant with state and federal laws.
It’s also important to know that methocarbamol may not act on the equine (or human) body the way people believe it does. Research shows it’s not acting directly on the contractile mechanism of the muscle, however it also shows it does interact with the interneurons of the spinal cord as a central nervous system depressant. This means it has sedation and relaxation properties on the brain and spine, not the muscle.
Because of its mechanism of action, side effects of methocarbamol use can include lethargy, muscle weakness, and ataxia. That could pose a safety risk for horses who are getting the medication too close to competition.
With that in mind, some practitioners have voiced concerns about the way they see the drug being used. Dr. Lori Bidwell, co-owner and founder of East-West Equine Sports Medicine, says she often sees clients using methocarbamol in concert with dexamethasone and a non-steroidal anti-inflammatory like phenylbutazone or Banamine.
“I'm seeing documentation of heavy medication loads in a lot of horses,” Bidwell said at a presentation before the USEF board of directors at the annual meeting in January. “The other mistake people make is they think they have to give it every single night before a competition.
“We’ve got to remind people that is not the way these drugs are meant to be used.”
Research has shown that if a horse is receiving only methocarbamol (without other drugs or supplements), the 12-hour withdrawal window is sufficient to avoid a positive test. Members have expressed concern to USEF about whether the administration of other medications or supplements can impact the metabolism or absorption of methocarbamol in horses. Given how many medications and supplements are out there for horses, it’s difficult to know whether some combinations could have an impact on methocarbamol. USEF staff are consulting with veterinarians to get a better understanding of the issue.
What does all of this mean?
“We’re not suggesting that people shouldn’t use methocarbamol, or that we’re going to prevent them from being able to use it at all,” said Dr. Schumacher. “But we are concerned about the risks involved in overuse, and about the recent increase in positives we’ve seen. We encourage people to work closely with their veterinarian to understand how it functions, when it makes sense for their horse, how to best use it, and how to source and administer it to avoid an accidental overage. We don’t want to see those any more than our members do.”