What counts as mental disorder, and the MHA guiding principles

What is the definition of mental disorder under the Mental Health Act?

Section 1(2) defines mental disorder as any disorder or disability of the mind. That single definition came in with the 2007 amendments, in force from 3 November 2008. The Act doesn't use mental illness as a legal test at all.

Whether someone has a disorder is a clinical question, answered by good clinical practice and accepted standards (Code 2.4). The Code's examples run from depression, schizophrenia and dementia to personality disorders, eating disorders, learning disabilities and autism (Code figure 1).

A diagnosis is never enough on its own. Every compulsory power adds criteria about the consequences of the disorder, and many disorders are unlikely ever to meet them (Code 2.6).

Difference isn't disorder

Nobody is mentally disordered just because of their political, religious or cultural beliefs, values or opinions (Code 2.8). The same goes for illegal, anti-social or 'immoral' behaviour. They count only where there are proper clinical grounds to see them as symptoms of a disorder.

Check your own assumptions too. The Code warns against diagnosing, or failing to diagnose, because of preconceptions or a failure to appreciate cultural and social differences (Code 2.7).

Is drug or alcohol dependence a mental disorder?

• Dependence alone isn't. There are no grounds to detain someone, or use any other compulsory measure, because of alcohol or drug dependence alone. Drugs here include solvents (s1(3); Code 2.10).
• A disorder that comes with it can be. If the criteria are met, you can use the Act for someone with a mental disorder who's also dependent. That's true even when the disorder results from the dependence (Code 2.11).
• Substance-related disorders count. Acute intoxication, withdrawal with delirium, psychotic disorders and organic disorders after prolonged use all remain mental disorders for the Act (Code 2.12).
• Treatment can include the dependence. Treatment under the Act can address alcohol or drug dependence if that's an appropriate part of treating the mental disorder that's the main focus (Code 2.13).

Learning disability and the qualification

In the Act, a learning disability is a state of arrested or incomplete development of the mind, including significant impairment of intelligence and social functioning (s1(4)). It's a mental disorder for every purpose, but a qualification applies to the powers listed in s1(2B).

Those are s3, guardianship, CTOs and their renewal, the court and transfer powers in ss35 to 38, 45A, 47, 48 and 51, and the tribunal's discharge tests. For them, the disability must be associated with abnormally aggressive or seriously irresponsible conduct. CTO recall is covered too (s1(2A)).

• Neither term is defined. It depends on the behaviour, its circumstances and how far it creates a serious risk to the person or others (Code 20.9). Look at persistence, severity, triggers, actual harm and the chance it recurs (Code 20.10 and 20.11).
• Bizarre isn't enough. Unusual behaviour isn't the same as abnormally aggressive or seriously irresponsible conduct (Code 20.12).
• Look for the cause. Challenging behaviour may come from pain, communication difficulties or an unmet physical, social or emotional need, not a mental disorder. Address that first (Code 20.13).
• Get a specialist view. Unless it's very urgent, the Code expects assessment by a consultant psychiatrist in learning disabilities and a formal specialist psychological assessment (Code 20.14).
• Write your reasons on the form. Say why the conduct is abnormally aggressive or seriously irresponsible, and why it relates to the learning disability rather than an unmet need (Code 20.16).

Because s2 isn't on the list, someone with a learning disability alone can be detained for assessment if they meet its criteria. Moving to section 3 then needs the qualification.

Is autism a mental disorder under the Mental Health Act?

Yes, and it has no equivalent qualification. Someone can meet the criteria for compulsory measures on autism alone, without aggressive or irresponsible conduct, though the Code expects that to be very rare (Code 2.17 and 20.19). It adds that compulsory treatment in hospital is rarely likely to help an autistic person (Code 20.20).

Personality disorders are treated exactly like any other mental disorder (Code 2.19). Make no assumptions either way. Decide on the person's needs, the risks and what can be done about them (Code 2.20).

Once commenced, the 2025 Act will require a psychiatric disorder, meaning a mental disorder other than autism or learning disability, for s3, CTOs and s3 renewals. Guardianship and Part 3, including CTOs after a hospital order, will still cover autism. They'll also cover learning disability with abnormally aggressive or seriously irresponsible conduct (2025 Act Sch 1).

What counts as medical treatment?

Medical treatment is wider than medication. It includes nursing, psychological intervention, and specialist mental health habilitation, rehabilitation and care (s145(1)). Its purpose must be to alleviate, or prevent a worsening of, the disorder or one of its symptoms or manifestations (s145(4)).

For s3, appropriate treatment must be available, meaning treatment that's appropriate given the nature and degree of the disorder and all the other circumstances (s3(4)). It doesn't have to cure. Preventing a worsening is enough.

What are the 5 guiding principles of the Mental Health Act?

Chapter 1 of the 2015 Code sets out five overarching principles. You should consider them whenever you make a decision about care, support or treatment under the Act (Code 1.1).

Are the principles law? The Code's legal status

The principles are guidance, but the Act requires them. Section 118 makes the Code include a statement of principles (s118(2A)). It must address wishes and feelings, diversity, minimising restrictions, involvement, non-discrimination, effectiveness, carers' views, wellbeing and safety, and public safety (s118(2B)).

Doctors, approved clinicians, hospital managers and staff, and AMHPs must have regard to the Code (s118(2D)). It's statutory guidance for local authorities too. For police and ambulance services it isn't statutory, though the Code says it will help them (Code introduction).

A departure from the Code can be challenged, so record your reasons clearly. Courts will look for a sufficiently convincing justification (Code introduction). Munjaz is the case behind the cogent reasons test.

How to use the principles in a decision

• They're equal. No principle outranks another. The weight each gets depends on the decision and its circumstances (Code 1.23).
• Lawful comes first. Every decision must be lawful, and that includes the Human Rights Act 1998 and the Equality Act 2010 (Code 1.22).
• Explain what goes against their wishes. Where a care plan decision is contrary to what the patient or others want, explain why and record it (Code 1.9).
• Avoid blanket restrictions. A rule for every patient in a setting needs a clear justification, the hospital managers' agreement and documented reasons. It should never be for the provider's convenience (Code 1.6).
• Write down any departure. Document and justify any decision to depart from the Code or a principle. CQC looks for this when it inspects (Code 1.24).

Mental Health Act principles vs Mental Capacity Act principles

People often mix the two sets up. Both have five, but they're different kinds of rule.

Both share the idea of the least restrictive option. The MCA's five are set out under the Mental Capacity Act 2005.

What the 2025 Act changes: four principles in the Act

The Mental Health Act 2025 rewrites s118(2B), so the next Code must include four principles and address set matters under each (2025 Act s1). It isn't in force. DHSC expects it in Phase 1, indicatively 2028 to 2029.

• Choice and autonomy. Involving patients in decision-making, and considering the views of carers and other interested parties.
• Least restriction. Minimising restrictions on liberty, so far as that's consistent with the patient's wellbeing and safety and with public safety.
• Therapeutic benefit. The effectiveness and appropriateness of treatment.
• The person as an individual. Treating patients with dignity and respect, and considering their attributes and past experiences.

Until then, the 2015 Code's five principles apply. What the 2025 Act changes, and when has the timetable.