Manchester City's Ivory Coast international defender Kolo Toure failed a drugs test after taking a diet pill belonging to his wife, Arsenal manager Arsene Wenger said on Friday.
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 14 million dollars (16.3mn euros) 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.