Laughter reduces arterial wall stiffness, which researchers suggest may relieve tension. And it lowered the risk of heartattack.
Hospital clowns improved lung function in patients with chronic obstructive pulmonary disease and 'genuine laughter' for a whole day could burn 2000 calories and lower the blood sugar in diabetics.
Laughter also enhanced fertility: 36 per cent of would-be mothers who were entertained by a clown after IVF and embryo transfer became pregnant compared with 20 per cent in the control group.
The researchers say that their review challenges the view that laughter can only be beneficial but do add that humour in any form carries a "low risk of harm and may be beneficial".
They conclude that it remains to be seen whether "sick jokes make you ill, dry wit causes dehydration or jokes in bad taste cause dysgeusia (distortion of sense of taste)".
“We categorised the effects as beneficial or harmful, a usually clear-cut distinction. Some effects, however, such as lowering the threshold for seduction, could not be unequivocally categorised,” said the authors.
“Some readers may ignore the benefits of laughter. That would be serious. Others may dismiss its harms. We call them the laughing cavalier.”
The findings were reported in the BMJ Christmas edition.