Medics thought Natalie Archibald was suffering from over-excitement when she collapsed after opening her presents on Christmas Day two years ago.
But she was later found to be suffering from the lung condition primary pulmonary hypertension and was referred to Great Ormond Street Hospital in London.
Doctors prescribed Viagra, better known as a treatment for male impotence, and the drug has since transformed her life.
Her mother Janis, from Musselburgh, near Edinburgh, said that she was surprised to learn the nature of the treatment that saved her daughter's life.
She added: "I've never seen her so happy. She can run, jump, skip, all the things her friends do. We have a wheelchair for her if she gets exhausted, but we've never had to use it, and I hope she grows out of it before we need to."
Mrs Archibald, 48, gave up her job as a legal secretary to look after Natalie after she began collapsing or turning blue on a regular basis following the first incident in 2006.
She said: "It was so frustrating because I knew there was something wrong with my wee girl but no-one could find out what it was.
"It got to the stage where we had almost given up trying to find out, until a doctor new to us went back into her records and carried out some more tests and diagnosed her.
"Within two weeks we were down to Great Ormond Street and the drugs had started, but it was a long fight to get there."
Natalie, a rare childhood sufferer of the condition, no longer experiences fainting episodes but the condition remains potentially fatal.
It is a progressive disorder that causes abnormally high blood pressure that affects arteries in the lungs. The Viagra works by opening the arteries and improving the blood flow.
