Robin Ferner of City Hospital Birmingham, in England, and colleagues conducted a review of studies that reported the benefits and harms of laughter. They used data published from 1946 to 2013.
The researchers identified benefits from laughter; harms from laughter and conditions causing pathological laughter.
Some medical conditions benefit from "unintentional," or 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, the Christmas edition of The British Medical Journal reported.
Laughter reduced arterial wall stiffness, and it lowered the risk of heart attack, so "reading the Christmas British Medical Journal could add years to your life," the researchers suggested.
However, clowns improved lung function in patients with chronic obstructive pulmonary disease and "genuine laughter" for a whole day could burn 2,000 calories and lower the blood sugar in people with diabetics, the review found.
Laughter also enhanced fertility: 36 percent of would-be mothers who were entertained by a clown after in vitro fertilization and embryo transfer became pregnant compared with 20 percent 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 can cause incontinence and hernias can occur after laughing.
The researchers concluded laughter can only be beneficial but do add that humor in any form carries a "low risk of harm and may be beneficial."