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Cats have been known to become ill after licking a pillowcase of someone using topical minoxidil, a treatment for hair loss.
The savvy cat owner had a hunch that the product her husband considered applying to his scalp to thicken his hair could be a problem for their pets. To be sure, she asked her veterinarian.
As a cat owner herself, the veterinarian, Dr. Kirsten Smith, was intimately familiar with feline behavior. "I know that they'll sleep on your pillow, they'll groom your head, they'll lick your head," she said.
Knowing these things, Smith agreed with her client that, yes, it was a safety risk to use hair-loss treatment containing the drug minoxidil in a household with cats.
The couple took the guidance to heart. "They elected not to bring that into their house," Smith recalled. "You know, they love their cats."
Their decision was prudent. Research published in the September/October 2021 edition of the Journal of the American Animal Hospital Association confirms that even very small exposures to minoxidil — mere licks — can have toxic-to-the-point-of-deadly effects in house pets, particularly cats.
The researchers reviewed 211 cases of topical minoxidil exposures and toxicoses in dogs and cats that occurred from 2001 through 2019. The study drew from the database of the Animal Poison Control Center operated by the American Society for the Prevention of Cruelty to Animals.
Among the 211 cases were those of 62 cats and 25 dogs that showed clinical signs of toxicosis; eight of the cats died.
The most common way cats were exposed was through "unintentional delivery," according to the study, such as by licking the owner's skin or pillowcase or being splashed with spilled medication. Dogs were most commonly exposed through their own "exploratory behavior," such as rummaging through trash.
Clinical signs, which started between one and 67 hours after exposure, included vomiting, drooling, hypotension (low blood pressure) and lethargy.
In many cases, particularly among the cats, the amount to which they were exposed was so low that it "could only be reported in licks." In one such case, the cat died. In another case requiring hospitalization and IV therapy, the exposure was a single drop that splashed on the cat, which the cat then licked off.
Dr. Tina Wismer, senior director of the ASPCA Animal Poison Control Center and one of three authors of the paper, said the center's electronic database, which dates to 2001, currently shows 390 cases of suspected minoxidil exposure — not a lot in a database of close to 3 million possible poisoning cases.
"The reason we notice these is that they tend to be severe," said Wismer, who also is a toxicology consultant at the Veterinary Information Network, an online community for the profession and parent of the VIN News Service. "We've got hundreds of thousands of chocolate cases, but most of those are mild vomiting. We come across items [like minoxidil] that are potentially deadly, and we tend to pull them out."
A dangerous sensitivity to a popular product
Minoxidil is a potent arterial vasodilator, meaning it's good at opening up, or dilating, blood vessels. That lowers blood pressure. Early on, minoxidil was studied as a drug to reduce hypertension (high blood pressure). Along the way, it was found to stimulate hair follicles, promoting hair growth. That chance discovery led to the development of the commercial product Rogaine, a topical form of minoxidil that debuted as a prescription drug in 1988. Rogaine, for men and women alike, became available over the counter in 1996, according to the product website.
Today, Rogaine and generic brands of minoxidil come in liquids, foams and shampoos that are massaged into the scalp and left on. The drug must be applied regularly to keep hair growing.
While minoxidil is not without side effects in people — including hypotension, tachycardia, myocardial infarction, elevated liver enzymes and fetal malformations — cats and dogs appear to be exquisitely sensitive to its cardiovascular effects.
"It's one of those, what we call, species differences," said Dr. Sharon Gwaltney-Brant, a veterinary toxicologist who worked at the ASPCA Animal Poison Control Center from 1997 to 2010, and is one of the authors of the paper on minoxidil toxicity in pets. Gwaltney-Brant also is a toxicology consultant at VIN.
Exactly what accounts for the species difference is unclear; it might be related to how the product is metabolized, Gwaltney-Brant said. What is known is that dogs and cats exposed to the drug become hypotensive. "Low blood pressure by itself isn't necessarily dangerous," she said. "However, there are some things going on in the heart that are causing heart damage ... and [patients] end up in heart failure. Nobody that I know has worked out why that happens."
Veterinarians generally are aware of the toxicity of minoxidil to pets, owing to earlier studies, including a paper in 2004 by Dr. Camille DeClementi (formerly at the ASCPA Animal Poison Control Center) and colleagues, who described the cases of two cats that were accidentally exposed to topical minoxidil and died. However, awareness among pet owners is spottier, Wismer believes.
"I think most people don't realize how much of a problem it can be," she said, especially for cats that like to snuggle with their owners in bed or groom their owners' heads.
People also may not appreciate how much animals examine their environment with their mouths, noted Gwaltney-Brant. "When we find something interesting, we pick it up and turn it around and look at it," she said. "Cats and dogs taste it."
Pet owners who use minoxidil may need to wear a cap and keep pets off their bed, if not entirely out of the bedroom. Easier said than done, Gwaltney-Brant acknowledged, but "when you have things that are potentially this dangerous to them in the house, you may need to change your lifestyle a little bit."
Anyone whose animal is exposed to minoxidil should seek veterinary care promptly, Wismer advised. "They should be monitored, watching their blood pressure, because we're going to see changes there before owners notice [other signs]," she said.
Poor circulation can lead to low oxygen levels reaching the heart and other organs, as well as pulmonary edema (fluid in the chest) due to vasodilation, she explained. Veterinarians can counter the effects with medications.
Keeping owners alert to household hazards
Minoxidil is just one example of potential drug hazards to pets. As the popularity of topical drugs increases, so do the risks of accidental exposures to pets, and children, too. Wismer and Gwaltney-Brant both mentioned as an example diclofenac, a non-steroidal anti-inflammatory drug in cream form that recently went over-the-counter under the brand name Voltaren, marketed for arthritis relief.
Other drugs that pet owners have been known to accidentally transfer to their pets are topical sex hormones — usually, estrogen and testosterone. Exposures can have a variety of disconcerting effects, from hair loss to enlarged sex organs and nipples to behavior changes.
Smith, the veterinarian who affirmed the savvy cat owner's instinct to avoid minoxidil, says it would be impossible to comprehensively list all possible hazards to pets in the home. So she makes a habit of discussing safety with clients of her Hollister, California, mobile practice in a way that encourages them to think analytically about what might pose a danger and how.
For example, Smith said, "I tell them there are a lot of things that can harm their cats — I always bring up lilies and cats. Birds, I'll bring up nonstick cookware." For dogs, she said, "I'll talk about onions, garlic, grapes, chocolate. Sometimes I talk about minoxidil, too."
And then there are physical hazards. "I'll talk about things like dental floss and electrical cords, just to kind of throw out some things that people might not have thought about," Smith said, "so people become cautious about what they bring in their homes."
VIN News Service commentaries are opinion pieces presenting insights, personal experiences and/or perspectives on topical issues by members of the veterinary community. To submit a commentary for consideration, email email@example.com.