People in their thirties who smoke cannabis have the "lungs of 80-year-olds" after less than 10 years of use, doctors have said.
Damian Mckeon, a consultant in respiratory medicine at Ysbyty Gwynedd in Wales, studied eight patients who smoked at least five joints per day, the Daily Mail reports.
In some cases, the patients' lungs were so badly damaged they had to be put on long-term oxygen therapy.
"We are seeing young people on the wards with the lungs of 80-year-olds after less than a decade of smoking cannabis and tobacco," McKeon said.
"Our study was in a rural region of North Wales but we believe these cases may represent the tip of the iceberg."
One patient is now waiting for a lung transplant, researchers heard at a British Thoracic Society conference.
Cannabis mixed with tobacco for use can cause more damage to the lungs because it is mostly smoked without a filter. The smoke is often inhaled for longer.
Adding to this, McKeon added: "Cannabis is far stronger these days and we are seeing the emergence of a new severe form of emphysema – which could lead to people struggling for breath for the rest of their life. We urgently need a detailed study across the UK which analyses the national picture of cannabis-use and lung disease."
Currently there are calls for cannabis to be decriminalised for personal use. A policy paper presented at the Liberal Democrat party conference in October said drug laws need radical changes, with more emphasis on treating the problem as a health matter rather than a criminal one.
Bernard Higgins, of the British Thoracic Society, said of the findings from Ysbyty Gwynedd: "This study is yet another small but persuasive piece of a jigsaw pointing to a real danger of regularly smoking joints.
"The government should monitor this emerging evidence carefully and take it into account when considering future drugs and smoking policy."