The Manchester City defender was suspended on Thursday pending further investigation after testing positive for a specified substance.
Wenger has spoken with the former Arsenal captain since the news broke and revealed what Toure had taken during a press conference on Friday.
“He wants to control his weight a little bit because that’s where he has some problems and he took the product off his wife,” said the Arsenal boss.
“Never trust your wife! That is how he was caught.”
Toure could face a lengthy ban and is currently suspended from playing for City.
Wenger signed the Ivory Coast international from domestic club Mimosas in 2002 and he went on to make 326 appearances in seven seasons with the Gunners.
Toure also became captain before making a £16million move to City in July 2009 and Wenger paid tribute to the 29-year-old’s character, insisting there was no malicious intent on his part.
“I think it is a mistake,” he added. “He was not cautious enough. I don’t think there’s a desire there to do something wrong and hide it.
“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?’.”
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Arsene Wenger – Kolo Toure Took Diet Pill