North America's increasing legalisation of cannabis is forcing sniffer dogs into early retirement.

As more states in the US and provinces in Canada legalise cannabis, dogs trained to sniff out narcotics are becoming redundant.

The highly-trained canines are causing more trouble than its worth for police forces because they cannot distinguish between cannabis and illegal drugs.

If the dogs sniff out drugs such as meth and heroin, a case can be thrown out of court if there was also cannabis in the stash.

All dealers have to do to get away with having illegal drugs is place a bag of marijuana among the narcotics and their arrests can be deemed illegitimate.


Image: The dogs have been trained to sniff out all drugs

At the beginning of November Michigan became the 10th US state to legalise recreational cannabis, while medical marijuana is now legal in 33 out of America's 50 states.

On 17 October Canada legalised recreational cannabis, with different laws on whether it can be smoked in public or only in private, depending on the province.

Canadian police said in July that 14 narcotics dogs were being eased into early retirement ahead of legalisation day.

Colorado and Washington were the first two states to legalise recreational cannabis in 2012, and a current court case against a sniffer dog has prompted forces to retire their dogs early.

Image: It costs nearly $10,000 (£7,790) to buy and train a police sniffer dog

Kilo, a drug-detection dog in Colorado, found drug traces in a man's truck, which turned out to be methamphetamine residue in a pipe.

Because Kilo was trained to find multiple drugs, including cannabis, judges said Kilo's nose was no longer reliable - even though there was no marijuana in the truck - so there was no legal ground to search it.

The Colorado Supreme Court is reviewing the decision but some departments in the state have decided they need to be ahead of the game so are retiring the dogs early.

New sniffer dogs are being trained to not detect cannabis, but each puppy costs about $6,000 (£4,670) to buy and $3,790 (£3,000) to train, which can take two to three years.

Tommy Klein, police chief in Rifle, Colorado, said Tulo, a yellow Labrador retriever has had to be retired, despite helping with more than 170 arrests in the town of 9,000.

"A dog can't tell you, 'Hey, I smell marijuana or 'I smell meth,'" he told The New York Times.

"They have the same behaviour for any drug that they've been trained on.

"If Tulo were to alert on a car, we no longer have probable cause for a search based on his alert alone."

Some states are deciding to not change their approach and take their chances in court.

In Michigan, police are still trying to determine what to do with their 50 dogs.

"We're still reviewing the law's impact on our operations and determining next steps," said state police spokeswoman Lori Dougovito.