The bit is a device used for communication between rider and horse that allows for control over the speed and direction of movement. However, Cook claims that “the bit method of control is invasive, physiologically contraindicated and counterproductive” which “often causes discomfort, pain and injury” in horses . Therefore, other methods such as bitless bridles should be considered as a communication method between rider and horse.
Quick & Warren-Smith  conducted a study in 2009 that compared bitted and bitless bridles through measuring cardiac and behavioural responses of horses undergoing foundation training. They found that horses with a bitted bridle exhibited more conflict behaviours such as chewing, opening the mouth, pawing and tail swishing rather than those wearing a bitless bridle . They also found that, during long-reining, the bitless horses exhibited more head lowering than that of the bitted horses . Bitted horses demonstrated discomfort that could indicate pain in response to the bit placed in their mouth. The study also indicated that horses with bitless bridles performed “at least as well as, if not better than, those in bitted bridles” demonstrating that the bit is an unnecessary addition to the bridle .
There is some debate over whether the use of bits may cause the death of some horses, particularly in the racing industry. In 2016, Cook  conducted another study on bit-induced asphyxia as a cause of sudden death in racehorses. He claims that catastrophic musculoskeletal injury and sudden death may be caused by bit-induced asphyxia and that the bit “is an ultimate cause of palatal instability, dynamic collapse of the upper respiratory tract, EIPH [exercise-induced pulmonary haemorrhage] and sudden death” . However, a similar study was conducted by Fretheim-Kelly and associates  in 2020 and they found that there was no evidence that a bit in the horse’s mouth could result in the dynamic collapse of the upper respiratory tract . Rather, they suggest that the head and neck angles caused by rein tension are what causes the dynamic collapse in susceptible horses . So, while the bit may not have any effect on the breathing capacity of racehorses, it is still important to consider the possible pain caused by bit use, particularly when other alternatives would relieve this potential issue.
However, there is also the concern that bitless bridles may cause some welfare issues. Robinson and Bye  recently conducted a study that analysed the noseband and headpiece pressure in bitless bridles in comparison to a bitted bridle. The study compared a snaffle bridle with a cavesson noseband, a side pull bitless bridle and a cross under bitless bridle . They discovered that all bridles have the potential to minimise blood flow to the naval tissues and that there were no performance advantages observed for any of the bridles . However, they found that the side pull bitless bridle caused significantly higher pressure on the naval tissues, which could result in nerve and tissue damage if maintained over time . They also found that the cross under bitless bridle seemed to create an extended head and neck position which could negatively affect musculoskeletal health . Therefore, while there are claims that using a bit to communicate with your horse can result in some negative welfare outcomes, emerging studies suggest that using a bitless bridle may also be of concern.
 Cook, W.R 1999, ‘Pathophysiology of bit control in the horse’, Journal of Equine Veterinary Service, vol. 19, no. 3, pp. 169 – 204
 Quick, J & Warren-Smith, A 2009, ‘Preliminary investigations of horses’ (Equus caballus) responses to different bridles during foundation training’, Journal of Veterinary Behaviour, vol. 4, pp. 169 – 17
 Cook, W.R 2016, ‘bit-induced asphyxia in the racehorse as a cause of sudden death’, Equine Veterinary Education, vol. 28, no. 7, pp. 405 – 409
 Fretheim-Kelly, Z, Fjordbakk, C, Fintl, C, Krontveit & Strand, E 2020,‘A bitless bridle does not limit or prevent dynamic laryngeal collapse’, Equine Veterinary Journal, vol. 53, pp. 44 – 50
 Robinson, N & Bye, T 2021, ‘Noseband and poll pressures underneath bitted and bitless bridles and the effects on equine locomotion’, Journal of Veterinary Behaviour, vol. 44, pp. 18 – 24