Obsessive‑compulsive disorder (OCD) is characterised by the presence of either obsessions or compulsions, but commonly both. The symptoms can cause significant functional impairment and/or distress. An obsession is defined as an unwanted intrusive thought, image or urge that repeatedly enters the person's mind. Compulsions are repetitive behaviours or mental acts that the person feels driven to perform. A compulsion can either be overt and observable by others, such as checking that a door is locked, or a covert mental act that cannot be observed, such as repeating a certain phrase in one's mind.

It is thought that 1% to 2% of the population have OCD, although some studies have estimated 2% to 3%.

Body dysmorphic disorder (BDD) is characterised by a preoccupation with an imagined defect in one's appearance, or in the case of a slight physical anomaly, the person's concern is markedly excessive. BDD is characterised by time‑consuming behaviours such as mirror gazing, comparing particular features to those of others, excessive camouflaging tactics to hide the defect, skin picking and reassurance seeking.

It is thought that 0.5% to 0.7% of the population have BDD.

The recommendations in this guideline were graded according to the quality of the evidence they were based on. The gradings are available in the full guideline and are not shown in this web version.