TMZ Sports

Ryan Braun's positive test for banned substances was caused by medication he's taking for a private medical issue -- NOT performance enhancing drugs ... this according to sources directly connected with Major League Baseball.

We're told the reigning National League MVP is adamant he has not taken drugs or steroids of any kind.

One source simply told us ... "The medication contained banned substances resulting in the positive test." We're also told prior to taking the dirty test, Braun had always tested negative for banned substances.

The nature of Braun's medical ailment is unclear.