Manchester City defender Kolo Toure failed a drugs test after taking a diet pill belonging to his wife, Arsenal manager Arsene Wenger says.

Wenger has spoken with the former Gunners captain since news broke of his positive test for a specified substance.

“He wants to control his weight a little bit because that’s where he has some problems and he took the product of his wife,” said Wenger.

Wenger added: “Never trust your wife! That is how he was caught.”

Toure could face a lengthy ban and is currently suspended from playing for City.

The 29-year-old moved to Eastlands in 2009 for 16.3 million euros ($A22.46 million) having spent seven seasons at Arsenal.

Wenger said Toure’s error was failing to check the legality of the product he took.

“I think it is a mistake. He was not cautious enough. I don’t think there’s a desire there to do something wrong and hide it,” said Wenger.

“It is a complete surprise because I had Kolo Toure here for years, I brought him here.

“He is a boy who has a clean life. He’s very honest living, always at home, a family man, and I don’t suspect him at all to have taken drugs to enhance his performances.

“I just think it is a mistake by forgetting to ask, ‘Can I take that?’.”

Toure has the right to request his B sample also be tested. The Ivorian would then get the opportunity to go before a personal hearing.