The mouth is at the heart of our daily life. However, oral health is often overlooked, especially in people with mental illness. The objectives of this review were to identify the factors that can increase their vulnerability toward oral diseases, to describe the consequences in terms of health and quality of life, and to explore solutions for preventing and managing oral diseases in this population. Why are patients with mental illnesses more at risk of poor oral health?The mental illnesses themselves, which can alter one's self-image and perception of the body, may decrease motor skills or disorganize daily life, contribute to a decrease in the practice of daily oral hygiene or in seeking healthcare services. The presence of vomit or gastroesophageal reflux stimulates the demineralization of dental surfaces. Addictive substances, the consumption of which is frequent in this population, particularly tobacco, have a direct impact on the oral mucosa and contribute to poor oral health as well as to a reduction of the effects of pharmacological treatments used to treat psychiatric pathologies. By stimulating the appetite, they promote snacking. By inhibiting the production of saliva, they prevent it from playing its role as a natural oral lubricant. It has a negative effect on self-cleaning dental surfaces, the neutralization of oral acidity and defensive power but also on food pre-digestion, chewing, swallowing and speaking. Finally, by inducing abnormal movements of the lips, tongue or jaw, problems of lip incompetence, breathing through the mouth or bruxism may develop. Difficulties in accessing oral care can also contribute to patients’ poor oral health. They may be related to mental pathology: the failure to keep appointments, refusal of care, anxiety, to the lack of information and understanding of their rights for health insurance and transportation to a dental office. They can also be linked to a stigmatizing or critical attitude or a rush to provide care from some dentists that can discourage the patient from continuing his/her oral follow-up. What are the consequences?Xerostomia has several consequences: difficulty in chewing, swallowing, speaking, wearing removable dentures and altered taste, bad breath, cracked lips, and burning mouth. The patient will then modify his/her diet and increase their consumption of soft drinks and soft foods, which adhere more to dental surfaces. Patients have an increased risk of carious lesions, erosion of the teeth, and periodontal lesions accentuated by smoking. People with substance addiction are roughly three times more likely to have periodontal pockets than control groups. People with eating disorders are five times more likely to have dental erosion, responsible for sensitive teeth than control groups due to vomiting or gastroesophageal reflux. Finally, the problem of dental caries is more severe in people suffering from mental illness: People with schizophrenia seem to be the most affected with 7.7 more tooth decay, missing and filled teeth on average than do control participants. Oral afflictions are responsible for acute and chronic pain and infection. Patients with severe mental illness are 2.8 times more likely to lose all their teeth than those in control groups. Edentulism has many repercussions for the patient's health and quality of life, (eating disorders, speech impediments, and deformed smiles, all of which can lead to low self-esteem and social isolation). What are the solutions?A set of measures can be proposed against xerostomia: frequent intake of unsweetened, non-acidic drinks, chewing sugar-free chewing gum with xylitol, salivary substitutes or salivary stimulants. Support measures to encourage patients to quit smoking, when possible and compatible with the patient's clinical condition, should be offered during psychiatric follow-up. Patients could be encouraged, as soon as their mental pathology has stabilized, to select a dentist and to consult him/her at least once a year for dental care and preventative measures and to become accustomed to receiving dental care. The continuum between psychiatric care and oral care is important, as is the training of medical teams. The number of dental treatments and/or the mental pathology of some patients can cause them to be uncooperative and may necessitate the use of a general anaesthesia during oral care, but this procedure has many limitations. For long-term health improvement and to avoid the accumulation of comorbidities, therapeutic patient education seems to be an appropriate complementary approach. Integrating oral health with psychiatric care would make it possible to encourage patients to develop habits of healthy eating and good oral hygiene, to seek dental care, and to acquire improved social behaviours.