A drug treatment that targets the underlying cause of cystic fibrosis (CF) has been shown to slow irreversible lung damage by more than 40%.
The drug, marketed as Orkambi, was tested for long-term effectiveness over more than two years in more than 1,000 trial patients, both children and adults.
New findings presented at the North American Cystic Fibrosis conference in Orlando, US, showed that the treatment reduced the rate of lung decline by 42% and caused a 35% drop in the number of serious infections.
Professor Stuart Elborn, from the Royal Brompton Hospital, London, who treated a number of the patients, said: "Long-term follow-up data has indicated that this treatment can prevent disease progression.
"Initially, we were able to show that you can make people a bit better. Now we're seeing exciting and reassuring long-term improvement."
He hoped the new results would tip the balance towards making the treatment more cost-effective.
The National Institute for Health and Care Excellence (Nice), which helps decide which new treatments should be available on the NHS in England and Wales, has recommended against Orkambi being offered free to patients.
"We hope this will lead to a further rethink about the long-term benefits," said Prof Elborn. "There are likely to be reductions in the cost of hospital visits and other treatments."
The study also showed a 40% reduction in hospitalisation rates, while death rates for patients were halved.
Another drug, Kalydeco, used to treat a different and much less common strain of CF, was also found to halve death rates.
In addition, so-called "real world" data on this drug, drawn from patient registries, showed that it roughly halved the number of UK patients having to receive lung transplants.
Both drugs contain molecules that alter the balance of salt and water in the lungs.
Orkambi is a combination of two molecules, ivacaftor and lumacaftor. Kalydeco only contains ivacaftor.
Prof Elborn said such treatments were needed while scientists searched for more permanent solutions to CF.
One route actively being explored is gene therapy designed to fix the underlying genetic fault that causes the disease.
A new experimental gene therapy treatment called QR-010 delivers a snippet of error-correcting genetic material via a nasal inhaler.
Newly reported results from a study of seven patients has produced evidence that the nasal spray is able to modify activity of the target gene.
"Progress in gene therapy is encouraging, but for the medium-term, drug treatment is the best way to go," said Prof Elborn.
About 10,500 people in the UK have CF, a genetic condition which affects one in every 2,500 babies born.
The condition clogs the lungs up with thick, sticky mucus, leading to repeated infections and ultimately shortening life.