The first 40 finishers of the 1,000-mile Iditarod Trail Sled Dog Race have tested clean of any illegal substances, including winner Lance Mackey, who has acknowledged using medical marijuana in past Iditarods.

Race officials say test results for the 15 remaining mushers are expected early next week.

Top mushers were tested late on the trail and others were tested after reaching the finish line in Nome.

According to race rules, anyone testing positive would face disqualification. The drug rule has existed in some form since 1984 but was never strictly enforced.

The Iditarod began testing sled dogs for prohibited substances in 1994.

Mackey, a throat cancer survivor who has won the race an unprecedented four consecutive times, has been open about his pot use.