In short: Mental health medicines work by adjusting how brain cells communicate, mainly through chemical messengers such as serotonin, noradrenaline, dopamine and GABA. They do not change who you are; the aim is to reduce symptoms so you can function and recover. Because people respond differently, finding the right medicine and dose often takes time, regular review and sometimes a switch.

This guide, written by a Ready Health prescribing pharmacist, explains in plain English how the main groups of mental health medicines are thought to work, why side effects happen and how to get the most from treatment.

How do brain cells communicate?

The brain contains around 86 billion nerve cells (neurons), each linked to many others. Messages travel along a neuron as a small electrical signal. At the end of the cell, the signal triggers the release of a chemical messenger (a neurotransmitter) into a tiny gap called a synapse. The messenger attaches to receptors on the next cell, a bit like a key fitting a lock, and passes the message on. Afterwards it is either taken back up into the first cell (reuptake) or broken down by enzymes.

Medicines can change this process at several points: by blocking reuptake so the messenger stays active longer, by slowing its breakdown, by stimulating receptors, or by blocking them.

Which brain chemicals are involved in mental health?

Messenger	Roles it is linked with	Examples of medicines that act on it
Serotonin	Mood, anxiety, sleep, appetite	SSRIs, SNRIs, some antipsychotics
Noradrenaline	Alertness, energy, focus, stress response	SNRIs, ADHD medicines
Dopamine	Motivation, reward, movement, how we judge what is important	Antipsychotics, ADHD stimulants
GABA	The main "calming" signal in the brain	Benzodiazepines, some sleep medicines, pregabalin (indirectly)
Glutamate	The main "activating" signal; learning and memory	Some mood stabilisers, newer treatments
Histamine and orexin	Wakefulness	Sedating antihistamines, some sleep medicines

Mental health conditions are not simply caused by "too much" or "too little" of one chemical. Genes, life events, stress, physical health, sleep and substance use all play a part, and medicines are one tool among several.

How do antidepressants work?

Selective serotonin reuptake inhibitors (SSRIs), such as sertraline, citalopram and fluoxetine, block the reuptake of serotonin. Serotonin and noradrenaline reuptake inhibitors (SNRIs), such as venlafaxine and duloxetine, act on both messengers. Other antidepressants, such as mirtazapine, work through different receptors.

The chemical change happens quickly, yet improvement in mood usually takes two to six weeks. This delay is thought to reflect slower adaptations, including changes in receptor sensitivity and in how brain networks connect. Early side effects such as nausea, headache or feeling more anxious often settle within the first couple of weeks.

How do antipsychotics work?

Antipsychotics mainly reduce dopamine signalling at specific receptors, which can lessen hallucinations, delusions and agitation. Many also act on serotonin, histamine, noradrenaline and acetylcholine receptors. This explains their different side effect profiles, which can include weight gain, sleepiness, restlessness, stiffness and changes in blood sugar or cholesterol, so physical health checks are part of treatment.

How do ADHD medicines work?

Stimulants (methylphenidate, lisdexamfetamine, dexamfetamine) increase dopamine and noradrenaline activity in attention and self-control networks and act within hours. Non-stimulants include atomoxetine, which blocks noradrenaline reuptake, and guanfacine, which acts on alpha-2A receptors; both build up over weeks.

How do anxiety and sleep medicines work?

Benzodiazepines and "Z-drugs" (such as zopiclone) enhance the calming effect of GABA. They work quickly but can cause tolerance and dependence, so UK guidance recommends only short courses. Pregabalin, sometimes used for generalised anxiety, reduces the release of several excitatory messengers. SSRIs and talking therapies are usually preferred for longer-term anxiety.

Why do people respond differently to the same medicine?

Genes: variation in liver enzymes such as CYP2D6 and CYP2C19 changes how quickly some medicines are cleared.
Body and health: age, weight, kidney and liver function, pregnancy and other illnesses all matter.
Other medicines: including over-the-counter and herbal products such as St John's wort, which interacts with many medicines.
Your goals: which symptoms matter most to you and which side effects you most want to avoid.

Adjusting the dose (titration) or switching medicine is a normal part of good care, not a sign of failure.

How can I get the best from my medicine?

Take it as prescribed and keep a simple note of benefits and side effects.
Make one change at a time where possible, so you can tell what is helping.
Look after the basics: regular sleep, daylight, activity, regular meals and limiting alcohol.
Check any new medicine, supplement or herbal remedy with a pharmacist.
Do not stop suddenly. Antidepressants can cause withdrawal symptoms that are sometimes severe or long-lasting, so NICE advises reducing gradually with support.
Attend regular reviews to decide together whether to continue, adjust or stop.

When should I get urgent help?

Seek same-day medical advice, or call 999 in an emergency, if you have chest pain, fainting, a rash with fever, severe agitation or confusion, a high temperature with muscle stiffness or twitching, or thoughts of harming yourself. You can also contact NHS 111 and select the mental health option, or call Samaritans on 116 123.

Frequently asked questions

How long until a mental health medicine starts working?

It varies. ADHD stimulants and some anxiety medicines act within hours, while antidepressants usually take two to six weeks to have their full effect.

Will I need to take it forever?

Not necessarily. Many people take a medicine for months or years, with planned reviews. When it is time to stop, the dose is usually reduced gradually.

Are mental health medicines addictive?

Most, including antidepressants and antipsychotics, are not addictive in the sense of causing cravings, although stopping suddenly can cause withdrawal symptoms. Benzodiazepines, Z-drugs, pregabalin and stimulants can cause dependence or be misused and are prescribed with extra care.

Will medicine change my personality?

It should not. If you feel emotionally numb or not like yourself, tell your prescriber, as a dose change or different medicine may help.

Key takeaways

Mental health medicines adjust communication between brain cells rather than changing who you are.
Different groups act on different messengers, which explains both their benefits and side effects.
Responses are individual, so titration, switching and regular review are normal.
Never stop a mental health medicine suddenly without advice.
Medicines work best alongside talking therapies, lifestyle support and good physical health.

Sources and further reading

Last reviewed: October 2026 by Shamir Patel, Independent Prescribing Pharmacist, Ready Health. This article is general information and does not replace advice from your own clinician.

If you would like to talk through your mental health or treatment, Ready Health in Standish, Wigan offers clinician appointments, counselling and ADHD clinic services. Any medicine is only prescribed after an individual assessment.