Iditarod officials announced this month that four dogs on Dallas Seavey's team tested positive for tramadol at the end of the 2017 race.
The Iditarod has listed tramadol as a prohibited drug for sled dogs since 2005, according to a race spokesman.
So, what is tramadol? And what does it do for dogs?
Tramadol is a synthetic opioid painkiller that became a federally regulated controlled substance in 2014.
It's a prescription-only drug used to manage acute and chronic pain in dogs and cats, according to veterinarians. Doctors prescribe it to humans for pain management as well.
Veterinarians say they use it to help dogs recover from surgery or short-term, painful injuries like knee tendon tears. But they also use it to help older dogs with pain from arthritis or cancer.
It's included in a list of drugs that the Iditarod prohibits mushers from using during the race.
Iditarod chief veterinarian Stuart Nelson said in an email that volunteer veterinarians with current U.S. Drug Enforcement Administration licenses are asked to bring "a small supply of tramadol" on the trail. He said the drug is used only to treat dropped dogs — dogs that mushers have left behind at a checkpoint often because they're sick or injured — that "could benefit from pain relief medication."
Veterinarians described tramadol as a small, white, bitter-tasting pill.
Seavey said he did not give his dogs tramadol and a race spokesman has said officials don't know how the drug got into the dogs' systems.
A testing team took urine samples from four of Seavey's dogs after they finished the 1,000-mile race to Nome, as was done with some dogs from each top 20 team. Iditarod officials said Seavey's four dogs tested positive for tramadol.
Seavey and many of his supporters say it wouldn't make sense to intentionally give a sled dog tramadol because, as with many painkillers, it can have sedative effects.
Some veterinarians say sedation or drowsiness is a potential side effect of tramadol. But they say the drug could also help mask pain, leaving the dog feeling better than it did before.
"I tell the owners, 'The dogs may be a little bit on the tired side or conversely (they) may feel so good because the pain is gone that they become too active,' " said Dr. James Hagee, a Trapper Creek-based veterinarian and chairman of the Alaska Board of Veterinary Examiners.
Margaret Eastman, a former head veterinarian for the 1,000-mile Yukon Quest sled dog race who now owns a Wisconsin animal hospital, said she has used tramadol in her practice to treat pet dogs, in part because it doesn't typically sedate them.
"It's not something where I would expect them to act sedated," Eastman said. "That's why I like it long-term. It leaves us with lucid patients, which is what everybody wants."
She said she prescribes the drug together with others like anti-inflammatories, so the dose is fairly low.
Eastman said she has no experience with the drug's use in dogs performing at the level of an endurance race like the Iditarod or the Quest.
Dr. Jamie Peyton, a veterinarian with the Integrative Medicine Service at the University of California, Davis, said tramadol is better suited to help dogs with chronic pain, like arthritis. She said tramadol can take two to four weeks to have an effect on chronic pain. The drug boosts serotonin levels, she said.
But its side effects can include sedation and gastrointestinal issues, Peyton said.
"One of the biggest side effects is sedation in dogs," she said. "But if the dog had been on it for a long time they get used to that sedation."
The American Kennel Club says "most dogs" tolerate tramadol well if given the proper dosage. It lists potential adverse reactions including drowsiness, vomiting, anxiety, dizziness and tremors.
Numerous other Alaska veterinarians contacted for this story from Kenai to Fairbanks either didn't return calls or said they didn't want to comment for this story.
A half-dozen Iditarod mushers interviewed Wednesday said they did not regularly use tramadol in their kennels. Some said they had never even heard about the drug until this month.
Longtime musher DeeDee Jonrowe of Willow said the only time she gave tramadol to one of her dogs was nearly a decade ago when her pet Labrador retriever, Parker, had lung cancer. She said Parker refused to eat the bitter pill on its own.
"I tried to hide it in those roasted chickens and that was the only thing Parker would eat at this point and he still would not eat it," she said. "I had to put it down his mouth."
Jonrowe said the veterinarian prescribed Parker tramadol soon before the dog died, and she did not notice any significant changes in its behavior.
Given her experience with Parker, she said she wasn't sure how an Iditarod sled dog would accidentally eat tramadol on the trail.
"Sled dogs are not eager to eat just anything any stranger puts in front of them," she said. "I'm not really sure how that would happen without someone physically putting some sort of medication down the dog's throat."
Anna Berington of Wasilla said she did not regularly use tramadol, but in either the 2016 or 2015 Iditarod, a race veterinarian gave her the drug for one of her dogs in Nome. She said the team had long since finished the race and she noticed one dog "seemed a little bit stiffer."
She said she asked a veterinarian if there was anything to help the dog, Floyd, rest better. She said she was given a plastic bag with some white pills and a note that said the drug was tramadol and that instructed her how to use it. She said she hadn't heard of the drug before.
"I gave the amount that was prescribed. I didn't notice anything weird — nothing noticeably different. Other than maybe he did rest a little bit," she said.
Tramadol has surfaced as an issue in at least one other sport: cycling. The U.S. Anti-Doping Agency calls the drug "powerful and dangerous" for its reported role in helping competitive cyclists get a late-stage boost as they push through pain toward the end of long races.
The drug is not banned by the World Anti-Doping Agency but is being monitored, according to the U.S. agency, which would like to see it banned.