Washing hands until they're raw, lining up cans in alphabetical order, hoarding every newspaper you’ve ever bought: welcome to the bizarre world of obsessive compulsive disorder (OCD).

Watching Jack Nicholson in his Oscar-winning performance as Melvin Udall in the 1997 hit movie, As Good as it Gets, had millions of people chuckling at the eccentric and neurotic antics he portrayed.

He negotiates his way through the streets of Manhattan painstakingly avoiding the cracks in the sidewalk. He washes his hands habitually and always with a new bar of soap, which he then immediately discards. He takes his own plastic cutlery to the same table of the same restaurant day in, day out for breakfast.

Entertaining to many, but to the people who suffer similar symptoms of OCD, such behaviour is a blight on their lives and the lives of those closest to them.

It interferes with their ability to function socially and occupationally because of the inordinate amount of time that is consumed by the symptoms: unwanted and recurring thoughts, feelings and sensations (the obsessive part), the irrational drive to do things (the compulsive part) and the severe anxiety that goes hand in hand with these symptoms.

It is an extremely debilitating condition that produces fear, apprehension, agitation and torment. It is also far more prevalent than most people imagine. OCD is the fourth most commonly diagnosed mental disorder, estimated to affect between 2% and 3% of the adult population.

Irrational, unreasonable and irresistible
It is generally accepted that there are four differing classes of OCD: obsessions that are aggressive, sexual, religious or harm-related, with checking compulsions; obsessions about symmetry that are associated with arranging and/or repeating compulsions (as with Melvin Udall); obsessions about contamination with cleaning compulsions (Udall, again); and symptoms of hoarding.

In spite of OCD sufferers' realisation that these thoughts are irrational and unreasonable, they are still irresistible. A sufferer carries out irrational behaviour in an attempt to remove the obsessive thoughts. But this delivers no more than temporary relief. Compounding the problem is that not performing these obsessive rituals can cause severe anxiety.

OCD is often accompanied by other psychiatric conditions including eating disorders and depression. And, because about 20% of OCD sufferers develop tics, there are suspicions that the condition could also be linked to Tourette's Syndrome.