Mind has issued a booklet to help people understand the condition more clearly and offer support to people close to sufferers.

It examines what paranoia is, the different types of the condition and what causes it.

It also explains who gets it - among the young it is more common in men, for example, whereas among older adults it is more common in women.

Different types

Paranoia varies from mild cases where the sufferer is aware their suspicions may be unfounded to extreme cases where they cannot tell the difference between fantasy and reality.

Although it is often associated with schizophrenia, it is associated with more than 80 medical conditions ranging from depression to ageing - for example, 32% of patients on geriatric wards are diagnosed with some form of paranoia.

There are different types of paranoia - some people develop paranoid jealousy, some believe their actions are being monitored or controlled while others think their life is in danger; they may think their food or the milk is being poisoned.

Several causes are listed, covering everything from genetic factors and childhood influences to drug abuse and significant life events.

The leaflet says treatment can be difficult, often because the patient often refuses to accept there is anything wrong with them, and that many paranoid people will be suspicious of medication.

However, psychotherapy, community-based projects and drugs also used in the treatment of schizophrenia can all help.

Families and friends have a significant role to play as well - but they must be prepared to take an objective view of the problem and seek help if necessary.

Understanding

A lot of what those close to the patient can do depends on them being able to separate what is caused by paranoia and what is not.

One instance the leaflet highlights is working out what can and cannot be controlled - a person may have no control over hearing voices but they can control how they interpret what the voices say, for example if they are recommending suicide.

It is also important to distinguish between what the person says that may be right - say, a bus driver being unfriendly - and what is clearly wrong - a milkman poisoning the milk.

One should not agree with things that are clearly wrong, but should also avoid being confrontational or too negative.

Useful aid

Marjorie Wallace, chief executive of the mental health charity SANE, said the leaflet could serve a useful purpose.

"Severe paranoia can be one of the most distressing experiences a person can undergo," she said.

"Imagine what it is like to walk down the street believing people are talking about you, or there's an assassin on every corner or that your food is being poisoned.

"The more information that is available about paranoia, the less frightening it will become to the person who experiences it and for the family or carers involved."

For some, a healthy trait

Dr Cosmo Hallstrom, a psychiatrist and medical director of the Charter Clinic in London, said it was important to talk to people with paranoia to understand what was causing it.

"Paranoia in itself is a personality trait and we all have it to some degree, and maybe a little bit of paranoia is not a bad thing," he said.