Toure was suspended by his club on Thursday after testing positive for a "specified substance" in an A-sample.
The World Anti-Doping Association said that a "specified substance" is something where there could be a "credible non-doping explanation" for its presence.
City have not said what Toure tested positive for.
Wenger, speaking in his regular pre-match news conference, said he believed it was a case of the player forgetting to check if he could take something.
"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," Wenger said.
"I just think it is a mistake by forgetting (to ask), 'Can I take that?'
"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. very honest-living, always at home, a family man, and I do not suspect him at all to have taken drugs to enhance his performances."
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