A paper in the British Medical Journal reviewed the literature on harms arising from laughter and produced a wide-ranging list of laughing-related dangers, from asthma attacks to cerebral tumors. The authors concluded "Laughter is not purely beneficial. The harms it can cause are immediate and dose related, the risks being highest for Homeric (uncontrollable) laughter."
Laughter is no joke—dangers include syncope, cardiac and oesophageal rupture, and protrusion of abdominal hernias (from side splitting laughter or laughing fit to burst), asthma attacks, interlobular emphysema, cataplexy, headaches, jaw dislocation, and stress incontinence (from laughing like a drain). Infectious laughter can disseminate real infection, which is potentially preventable by laughing up your sleeve. As a side effect of our search for side effects, we also list pathological causes of laughter, among them epilepsy (gelastic seizures), cerebral tumours, Angelman’s syndrome, strokes, multiple sclerosis, and amyotrophic lateral sclerosis or motor neuron disease.
Laughter and MIRTH (Methodical Investigation of Risibility, Therapeutic and Harmful): narrative synthesis [R E Ferner and J K Aronson/British Medical Journal]