A noticeable change in someone’s behaviour, mood, or personality can be frightening—especially when it seems “out of character.” Most of the time, these changes are linked to stress, sleep deprivation, medication side effects, mental health struggles, hormones, or illness. But sometimes, behaviour change can be a symptom of a medical problem affecting the brain, including (rarely) a brain tumour.
This guide explains when behaviour changes might signal a health problem, the tumour-related warning signs, and what to do next—without panic.
Important: Behaviour change alone does not mean cancer. The goal is to recognise red flags and get the right help.
Why illness can change behaviour (in simple terms)
Your brain controls emotion, judgment, impulse control, speech, memory and personality. A problem that affects brain function—such as inflammation, infection, bleeding, stroke, low vitamin levels, thyroid imbalance, medication effects, or a tumour—can sometimes show up first as a behaviour or personality shift.
Certain tumours, especially those affecting the frontal lobes, may cause personality and behaviour changes (for example, apathy, forgetfulness, reduced interest in normal activities).
Brain tumours: can they cause personality or behaviour changes?
Yes—sometimes. The NHS lists “mental or behavioural changes” (including memory problems or personality change) among common symptoms of brain tumours, alongside headaches, seizures, nausea/vomiting, drowsiness, weakness, and vision/speech issues.
It’s also important to know that benign (non-cancerous) brain tumours can still cause symptoms by pressing on brain tissue. The NHS notes they can include changes in behaviour or personality, confusion, and memory/concentration problems.
The American Cancer Society also includes personality or behaviour changes among possible symptoms of adult brain tumours.
10 Red Flags: When Behaviour Change Could Be Medical (Including a Tumour)
These signs don’t diagnose a tumour—think of them as reasons to get checked (especially if new, worsening, or persistent).
1) The change is new and persistent
A clear “before vs after” shift that lasts days/weeks (not just a bad day) deserves attention—especially if it’s getting worse.
2) Confusion or acting “not themselves,” suddenly
Sudden confusion can be serious. The NHS advises going to A&E or calling 999 if someone suddenly becomes confused.
3) New memory problems, disorientation, or trouble finding words
For example: forgetting familiar routes, repeating questions, struggling with everyday tasks.
4) A marked change in personality
Examples: unusual irritability, emotional flatness, impulsive/risky behaviour, loss of inhibition, or strong apathy—especially if family members say the person seems “different.” (This can occur with frontal lobe involvement.)
5) Behaviour change plus headaches that are new, worsening, or different
Brain tumours can cause headaches, but headaches are common for many harmless reasons too. What matters is the pattern and whether other symptoms appear alongside.
6) Behaviour change plus seizures (fits) — especially first-time
Seizures are a listed symptom of brain tumours.
In the UK, call 999 if it’s the first seizure, or if it lasts over 5 minutes, repeats, or the person doesn’t fully recover.
7) Behaviour change plus nausea/vomiting and increasing sleepiness
These can be related to raised pressure in the skull from different causes, including a tumour.
8) Behaviour change plus vision or speech problems
Blurred/double vision, new trouble speaking, or slurred words can be neurological warning signs.
9) Behaviour change plus weakness, numbness, or clumsiness (especially one-sided)
Progressive weakness/paralysis on one side is listed by the NHS as a possible brain tumour symptom—and it can also be stroke-related.
10) A “cluster” of symptoms that steadily worsens
One symptom is rarely the whole story. The more you see a cluster (behaviour change + headaches + seizures + vision/speech + weakness), the more important it is to seek medical assessment.
More common causes of behaviour/personality change (not tumours)
Behaviour change often has non-cancer explanations. A GP will typically consider:
- Delirium / sudden confusion (infection, dehydration, medication effects) — treat as urgent if sudden.
- Stroke/TIA (FAST signs: face drooping, arm weakness, speech problems/confusion) — call 999.
- Meningitis/sepsis (fever, vomiting, confusion, drowsiness, seizures) — call 999 if suspected.
- Thyroid disorders (low mood, difficulty thinking; or anxiety/irritability and mood swings).
- Vitamin B12 deficiency (can cause psychological problems including depression, anxiety, confusion).
- Medication/substance effects, sleep problems, severe stress, depression/anxiety, hormonal changes (including perimenopause/menopause)
What to do next (practical steps)
If it’s sudden or severe: treat it as urgent
Call 999 or go to A&E now if any of the following apply:
- sudden confusion or delirium
- stroke symptoms (FAST)
- first-time seizure or seizure lasting >5 minutes
- suspected meningitis/sepsis
- sudden “thunderclap” severe headache (can be a brain bleed/aneurysm)
If it’s not an emergency: book a GP appointment soon
Especially if the change is:
- new and persistent
- worsening over time
- happening with headaches, memory issues, sleepiness, or neurological symptoms
Bring a “symptom note” (it helps doctors a lot)
Write down:
- when it started (exact date if possible)
- what changed (examples of behaviour)
- any triggers (infection, stress, new medication, alcohol, poor sleep)
- associated symptoms (headache, nausea, vision changes, weakness, seizures)
- any weight loss, fevers, night sweats, new pain
- all medicines and supplements
FAQ
Can a tumour elsewhere in the body cause behaviour change?
It’s less direct. Behaviour/personality changes are more commonly linked to brain-related problems (including tumours in the brain or cancer that has spread there), or to body-wide issues like hormones, infection, or vitamin deficiencies. A GP can assess your overall symptoms and decide what tests are appropriate.
What if the person refuses to see a doctor?
Focus on safety and specifics: “I’ve noticed X started on Y date, and it’s getting worse.” If there’s sudden confusion, seizure, or stroke signs, seek urgent help even if they minimise it.
Safety note
This article is for information only and cannot diagnose a tumour or any condition. If you notice persistent or worrying behaviour changes—especially with confusion, seizures, severe headache, vision/speech changes, or one-sided weakness—contact your GP or seek urgent medical help.
