The 35-year-old star walked in on the cage fighter using the hormone, which is used to help 11 year old Harvey - Katie's child with Dwight Yorke - with his Prader-Willi syndrome, a disease that causes poor muscle tone and cognitive disorders, at their home in Surrey, South East England.
It is believed the incident was a major factor in the couple's split two years ago after their rocky 11-month marriage suffered two failed IVF attempts and revelations that Alex was a frequent cross-dresser.
A source told Now magazine: ''He was also on a prescribed growth hormone from the doctor to help him recover after a fight. He says Kate had kept both sets of phials in the same fridge. Alex swears it was a mistake that only happened a couple of times.
''He's adamant it wasn't deliberate. Alex insists Harvey was never put at risk as he had lots of supplies. He didn't understand why Kate went so mad. He claims he had no clue what he'd done.''
However, the former model - who also has son Junior, eight, and daughter Princess Tiaamii, six, with her first husband Peter Andre, and newborn son Jett with current spouse Kieran Hayler - was fuming with Alex after catching him and believes he should have known the difference between the two drugs.
Alex confirmed the incident to the publication, saying: ''It was an error on my part that I didn't check the labels properly. I never expected Kate to place Harvey's meds next to mine. On the face of it, they both looked identical.''
The revelation comes as Katie prepares to release her latest autobiography 'Love, Lipstick and Lies' later this month.
'Wilson', starring both Dern and Harrelson, is released in U.S. cinemas on March 24th.
Madonna adopted twins Esther and Stella from Malawi earlier this month.