A combination of two drugs has been used to treat the most common form of cystic fibrosis, with researchers optimistic about the trials.

A combination of two drugs has shown promise toward improving the health of people with the most common form of the incurable lung disease known as cystic fibrosis, researchers say.

Patients treated with two medications - lumacaftor and ivacaftor - saw "significant" gains in their ability to breathe and fewer lung infections than those taking a placebo, according to the results of two international clinical trials published on Sunday in the New England Journal of Medicine.

The randomised controlled trials included 1108 people, age 12 and older, who were treated for six months.

"These groundbreaking findings will benefit around 15,000 patients in US alone," said Susanna McColley, one of the study's authors and a professor of pediatrics at Northwestern University's Feinberg School of Medicine.

About 75,000 people in Europe, North America and Australia suffer from cystic fibrosis, which is caused by genetic mutations.

The disease causes the body to overproduce thick mucus that leads to chronic lung infections and pancreatic problems.