A teenage girl in Oklahoma has been left with no brain activity for days as her dad said she attempted a dangerous online challenge, according to her father. 

Leah Presson, 15, is fighting for her life in an Oklahoma City hospital after allegedly taking a deadly dose of Benadryl for a 'challenge' on social media. 

'I just want everybody to be aware of where they can educate their kids,' her father, Richard Presson, told KFOR. 

'These challenges are just silly, and they need to be educated so it doesn't happen again.' 

Richard described Leah as the most giving person with a contagious laugh, the outlet reported. 

'Her eyes. Her smile. The way she can just do her hair. Very characteristic, her personality,' the emotional father said, fighting back tears. 

On Sunday, Richard announced that doctors had pronounced his daughter brain dead and that the hospital was preparing to discontinue life support, but said he continued holding on to hope. 

'We're still waiting on a miracle, even though some have given up, Amber and I are still here waiting on this miracle to happen,' he said on social media. 

'We have chosen to donate her organs because she is the kind of girl that would have given anything to anybody. She loved Grey's Anatomy and is able to possibly save up to 90 lives with her little body.'

Richard added that the hospital had 72 hours to harvest Leah's organs, and there would be an honor walk for her at Integris Baptist hospital where she had been for several days.

'She is still here at the hospital, we're not giving up on our little Leah,' he said. 

'I tell her, Leah keep fighting and stay strong. You got this. You're a fighter,' Richard told the outlet. 

According to Dr Ryan Brown, a children's doctor at OU Health, the 'Benadryl Challenge' is known to doctors across the state and it is considered extremely dangerous, KFOR reported. 

'When you take the Benadryl or diphenhydramine in excess... it can lead to seizures and cardiac arrhythmias. Sometimes those cardiac arrhythmias lead to cardiac arrest, which can lead to no blood to the brain, which can lead to brain death,' Brown told the outlet. 

Presson said his daughter was rushed to the emergency room when her family saw what at the time they believed to be Asthma symptoms.

'I was met by the chaplain, and they said, "We need to pray,"' Richard said. 

'I don't even want to think about a funeral because I feel like there's still hope and everybody's like, giving up too soon. I believe in miracles, that she's definitely a miracle baby.'

According to Richard, Leah had attempted the challenge before and her symptoms matched those linked to it. 

'If your child is acting strange, look around,' Dr Brown the outlet. 'Make sure that there's not an empty pill bottle or something in the vicinity that may lead you to a clue as to what may have been going on.'

According to Kaci Williamson, Leah's mother, she was pronounced brain dead on Sunday afternoon at 2.25pm.