Smoking is linked to a significantly increased risk of developing type 2 diabetes, US research suggests.
Proving a cause and effect is very difficult
The University of Lausanne looked at 25 studies involving 1.2m patients.
They found smokers had a 44% increased risk of type 2 diabetes compared with non-smokers - with the risk rising with the number of cigarettes smoked.
The Journal of the American Medical Association study found the increased risk for those who smoked at least 20 cigarettes a day rose to 61%.
For lighter smokers the risk was 29% higher than for a non-smoker.
The increased risk of developing diabetes in former smokers was 23%.
The researchers said: "We conclude that the relevant question should no longer be whether this association exists, but rather whether this established connection is causal."
They admitted that the research did not prove that smoking contributed to the development of diabetes.
But they suggest the fact that people who smoked most heavily were most at risk is significant.
Previous research has linked smoking to insulin resistance - a condition which often leads to diabetes.
However, proving a link is very difficult because smokers tend to indulge in other unhealthy habits, such as not exercising enough and eating unhealthy foods.
About three in 100 people in the UK aged over 40 and about 10 in 100 people over 65 have type 2 diabetes.
It is more common in people who are overweight or obese and also tends to run in families.
Douglas Smallwood, of the charity Diabetes UK, said: "This is interesting research. Smoking may be one of the pieces missing from the diabetes jigsaw which could help us to identify who is at higher risk.
"We already know that certain lifestyle factors such as being overweight or leading a sedentary lifestyle increase the risk of developing type 2 diabetes.
"Type 2 diabetes is increasing at an alarming rate with 100,000 people diagnosed every year.
"The more we can find out about the exact causes of the condition the better chance we have of slowing down this growth."