 "Laughter may not be the best medicine")
Researchers from Birmingham and Oxford, in the UK, reviewed the reported benefits and harms of laughter. They used data published between 1946 and 2013.
Researchers identified benefits from laughter; harms from laughter; and conditions causing pathological laughter.
Some conditions benefit from 'unintentional' (Duchenne) laughter. Laughter can increase pain thresholds although hospital clowns had no impact on distress in children undergoing minor surgery (even though they were in stitches), researchers found.
Laughter reduces arterial wall stiffness, which the researchers suggest may relieve tension. And it lowered the risk of heart attack, they said.
Clowns improved lung function in patients with COPD 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.
However, laughter can also have adverse effects. One woman with racing heart syndrome collapsed and died after a period of intense laughter and laughing 'fit to burst' was found to cause possible heart rupture or a torn gullet.
A quick intake of breath during laughing can cause inhalation of foreign bodies and can provoke an asthma attack. Laughing like a drain can cause incontinence. And hernias can occur after laughing.
The authors' list conditions that cause pathological laughter and this may help in diagnosis. Epileptic seizures ("gelastic seizures") are the most common cause.
The researchers said 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 concluded 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)".
The study is published in the British Medical Journal.