Slap fighting can cause brain damage.
Slap fighting can have serious health implications
The combat sport is becoming increasingly popular and sees competitors taking turns to deliver full-force strikes to their opponent's cheek but medics are fearful of the impact it could have on health.
No headgear is allowed and flinching is banned but an expert has poured scorn on the sport's billing as "the ultimate test of toughness".
Nitin Agarwal, from the University of Pittsburgh's School of Medicine, said: "The safety of this... I don't want to call it a sport... the safety of this dangerous activity really needs to be called into question."
Agarwal suggested that it is strange that slap fighting is allowed to take place in an era where sports are doing more to tackle the risks of concussion in athletes.
He said: "Repetitive concussions, we know, have a host of manifestations. Are people going to be suffering from cognitive deterioration? Are they going to be suffering from ocular or eye and ear problems? Problems with gait and balance? We are really concerned about the long-term ramifactions."