"Mel is totally in control at the moment because she has to regularly take stringent drug and alcohol tests as part of her custody battle."
The latest test, which was reportedly taken at the weekend, just before the stair fall came back clear.
"This was her 15th drug and alcohol test and she has cleared them all," the source said.
"She's not going to accept any sort of slur on her reputation because Stephen wants to try and make her look bad. The results are there to see in black and white."
The singer was forced to cancel a signing of her book Brutally Honest, due to her accident.
However she was surrounded by her Spice Girls bandmates in hospital, posting a photo of the group to social media with the caption: "When your besties show up to hospital with hugs, laughter and A LOT of love."
A source close to Mel told the Sun her injuries would not impact preparations for the Spice Girls tour next year.
