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, adding: "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 having spent seven seasons at Arsenal. Wenger spoke of his character in glowing terms and insisted 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?'."