The side effects are little known

Doctors found smokers developed the unpleasant side effects up to two weeks after quitting.

And they say many ex-smokers could be deterred from staying the course unless they are told what to expect.

The study, published in the British Medical Journal's Tobacco Control, studied 174 smokers at one, two and six weeks.

Symptoms

After one or two weeks' abstinence the quitters were more likely to have cold symptoms, including sore throats, coughing and sneezing.

During the first week of stopping around a third of those studied had three or more symptoms.

It is an unexpected side effect of giving up smoking Amanda Sandford of ASH

They found mouth ulcers were significantly more likely during the first fortnight. After six weeks all the symptoms had subsided.

The authors say the increase in mouth ulcers could be related to the loss of the antibacterial effect of smoking and the rise in cold symptoms could be linked to a short-term depression in the immune system.

Positive move

The scientists, from the Department of Psychology, at St George's Hospital Medical School, said they did not fully understand the impact of the symptoms on the relapse rates, but said they might be linked to people taking up smoking again.

Dr Michael Ussher, said: "This study is the first to provide clear evidence for an increase in reports of cold systems and mouth ulcers following smoking cessation.

"Being psychologically prepared for these effects may reduce their impact on the attempt to stop smoking."

Amanda Sandford, research manager for ASH, Action Smoking Health, said people should not be discouraged by the temporary side effects.

"It is an unexpected side effect of giving up smoking.

"It is important that people are warned about it.

"You just need to give smokers a reassurance that it is a positive thing to give up smoking."

But she said that anyone whose symptoms lasted for a long time should contact their GP.