Two decades ago, if you mentioned dimethyl sulfoxide (DMSO) around the barn you may have gotten puzzled looks in response. Back then, this industrial solvent turned anti- inflammatory therapy was relatively new to the horse world, and even if people had heard of it they viewed it as an unusual or even mysterious option. Since then, DMSO has gone mainstream. It is approved by the Food and Drug Administration (FDA) for topical use on horses, but veterinarians regularly use the chemical “off-label” in other ways to treat a wide array of inflammatory conditions, from laminitis to neurological problems. When the DMSO’s distinctive garlicky smell wafts through the barn these days, people are more likely to nod knowingly than wonder what’s going on.
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Nonetheless, DMSO is a little like aspirin—something that many people use without necessarily thinking too much about how it works. But learning more about DMSO may help you appreciate how it can be useful in maintaining your horse’s health and comfort. So here are seven things you might not know about DMSO:
1. The therapeutic properties of DMSO were discovered more or less by accident.
A byproduct of paper production, DMSO was first developed as an industrial solvent. As people worked with the chemical, they noticed that if they spilled a bit of DMSO on their hands a distinct garlicky taste would be on their tongue shortly thereafter. This intrigued chemists, who began researching how the liquid could so quickly pass through skin and mucosa, but the work was generally limited to exploring non-medical applications.
Then, in the 1960s, Stanley Jacob, MD, began investigating DMSO as a preservative for organs destined for transplants. Work done in his laboratory soon sparked a rush of research into DMSO’s possible medical uses.
By 1965, however, the FDA closed down clinical trials citing safety concerns. Since then, DMSO has been approved for some specific applications. In human medicine, these include use as an organ preservative and as treatment for a bladder disease called interstitial cystitis. In 1970 it was approved for topical use in horses and dogs.
2. DMSO’s anti-inflammatory properties come primarily from its antioxidant action.
DMSO is classified as a nonsteroidal anti-inflammatory drug (NSAID), just like phenylbutazone (“bute”). Unlike bute, however, DMSO controls inflammation primarily by acting as an antioxidant.
You may be more familiar with anti-oxidant sources in nutrition, such as vitamin E or lycopene, but DMSO has a molecular structure that allows it to function in the same way. Specifically, DMSO binds with “free radicals,” which are oxygen compounds—leftovers from normal biochemical reactions—that damage or destroy healthy cells. These free radicals are often the byproduct of inflammation and, in turn, cause more swelling and inflammation as they accumulate. DMSO is a free radical scavenger that slows or halts this process.
When used to control inflammation in strains and other soft-tissue injuries, DMSO is usually applied topically. However, it may also be injected into a target area when an injury occurs in particularly dense tissue, such as a bowed tendon, or the site is difficult to reach. A veterinarian may administer DMSO orally or intravenously in the early stages of laminitis to try to slow or halt the inflammatory cascade.
3. DMSO draws fluid from tissues.
DMSO is a hygroscopic compound, meaning it attracts water. This property makes the compound especially versatile. For instance, it can reduce edema in swollen limbs and is often used as part of a “sweat” to combat stocking up. DMSO can also reduce swelling in the brain and spinal cord, which can be literally lifesaving in diseases like West Nile encephalitis. DMSO may be used to draw fluids out of the lungs in cases of acute pulmonary edema.
Because it pulls water from tissues, DMSO has a diuretic effect, meaning it makes a horse urinate more. This can help flush toxins from the body more quickly. With this action in mind, DMSO is often given intravenously in the treatment of cantharidin poisoning (blister beetle toxicity), to lessen the effect of the toxin on the kidneys and intestinal tract. After episodes of tying up, DMSO may help horses eliminate waste products of muscle breakdown through their urine more quickly. The diuretic action of DMSO, however, can make it unsafe for horses who are dehydrated or in shock. It can further dehydrate these animals or dangerously lower their blood pressure.
4. DMSO can carry other substances through the skin.
DMSO’s molecular structure allows many substances to dissolve completely within it. It also allows the chemical to transport these dissolved substances through cell membranes without damaging them, even if these substances wouldn’t be able to pass through on their own.
For instance, treating rainrot or other skin infections can be difficult because the responsible organisms are buried deep under the skin or crusty, painful scabs. However, a mixture combining antibacterial medication with DMSO can pass through the skin and reach the affected area. For the same reason, DMSO is often added to antifungal medications for treatment of eye conditions and sometimes to steroids for targeted, topical anti-inflammatory treatment.
It’s important, of course, to avoid inadvertently mixing DMSO with potentially toxic substances. You wouldn’t want fly spray crossing into your horse’s bloodstream, for example, so take care to avoid applying DMSO to your horse if he has recently been sprayed. Likewise, avoid mixing DMSO with substances that could be toxic if ingested, such as organophosphates or mercury salt. Also be mindful that the effects of some drugs, such as corticosteroids and atropine, are intensified when mixed with DMSO, so doing so needs to be done with caution under a veterinarian’s direction.
5. DMSO may provide pain relief on its own.
Although it’s commonly mixed with compounds to provide pain relief, some studies suggest that DMSO alone has analgesic properties. Research shows that DMSO slows or blocks conduction of impulses along nerve cells, which in effect reduces pain from musculoskeletal injuries, postoperative incisions and other sources. Relief is only temporary—lasting up to a few hours—because as the DMSO dissipates, normal nerve function returns. It can be combined with other pain-relieving drugs, however, to extend the analgesic action.
If it seems like DMSO has a variety of purported actions, that’s true. It’s nothing if not versatile. Some applications of DMSO combine all of these: For instance, it is often used in surgical colic cases to reduce the risk of tissue adhesions due to inflammation and poor circulation; some surgeons think that it may also provide some pain relief in the hours following surgery.
6. DMSO is a prohibited substance in some sports.
DMSO isn’t technically a medication, but various competition organizations treat it as one when it comes to drug testing.
The Fédération Equestre Internationale (FEI) classifies DMSO as a “controlled” prohibited substance, meaning that although it may have some therapeutic value, DMSO has the potential to affect a horse’s performance and its use is forbidden or limited. As of January 2018, the threshold of DMSO allowed by FEI was 15 micrograms per milliliter in urine or one microgram per milliliter in plasma. The Jockey Club allows 10 micrograms per milliliter of plasma. If you compete with your horse, check any governing association rules regarding DMSO use.
Also keep in mind that because DMSO can move other materials through the skin, combining it with other medications could result in a violation of thresholds for both. For instance, mixing DMSO with the topical NSAID Surpass (diclofenac) can cause an increase of the medication in the bloodstream, leading to a disqualifying test. The medication guidelines for the United States Equestrian Federation state, “Do not apply diclofenac cream in combination with any other topical preparations including DMSO, nitrofurazone, or liniments.”
7. DMSO use in horses hasn’t been studied extensively.
A search of a research database will turn up a good sampling of peer-reviewed papers on DMSO use in horses, but the compound has gotten far less scientific scrutiny than have medications developed specifically for therapeutic applications. Some studies about DMSO’s effects contradict each other, particularly in regard to its use for treating arthritis.
Scant information is available regarding dosing. Often veterinarians rely on their own experiences and those of their colleagues in deciding when and how to use DMSO. That is why it’s important to let your veterinarian lead the way when it comes to treating your horse with this compound.
Whether applied topically, orally, intravenously or by injection, DMSO requires careful handling. But its versatility means that if you haven’t used it so far, you probably will eventually. If you do, keep these basic facts in mind to make sure your horse benefits fully from this unusual preparation.