The FA issued a statement on its website saying Toure had failed the test after taking a diet pill that was not intended to enhance his performance.
The Ivorian international has been suspended from all football for six months from the date of the start of his provisional suspension on March 2 2011.
The FA's regulatory commission said Toure admitted the offence after taking "water tablets" obtained through his wife.
The commission found there was no intent "to enhance sporting performance or to mask the use of a performance enhancing substance.
"He was at fault in the limited and perfunctory efforts he made in relation to the water tablets; the checks he made in relation to those tablets were inadequate and fell some way below what it would be reasonable to expect of a professional footballer in these circumstances," the statement said.
Toure's six-month ban means he can to return to action in the first week of September and is likely to miss only a handful of matches in the new Premier League season which kicks off in mid-August.
"This has been a difficult period for me, and I am sad to have missed the team's triumph of securing Champions League football for Manchester City and also the FA Cup victory at Wembley," Toure told the club's website.
"But I am relieved that I will be able to return to football in September and thank the FA's commission for their understanding about my case in coming to their decision."
Toure, whose brother Yaya scored the only goal in City's FA Cup Final win over Stoke City at Wembley earlier this month, will be target-tested for a two-year period, the FA said.
