Bipolar disorder vs borderline personality disorder: how they differ

The simplest way to separate the two is by pattern and trigger. In bipolar disorder, mood moves into recognisable episodes of high or low mood that persist for days or weeks, often with changes in sleep, energy and activity, and then settles. In borderline personality disorder, emotions are intense and unpredictable, can swing over a short space of time and sit alongside unstable relationships, in a pattern that usually emerges in adolescence.

They also sit in different diagnostic families: bipolar disorder is a mood disorder, whereas BPD is a personality disorder, the term for a long-standing pattern of thinking, feeling and relating that differs significantly from the average. A specialist assessment, not a self-test, is how the two are separated, and some people receive both diagnoses.

The NHS says the main symptom of bipolar disorder is extreme changes to mood, with either high moods (mania or hypomania) or low moods (depression). High mood can include feeling very happy and excited or irritable, having lots of energy and little need for sleep, racing thoughts, talking fast, feeling overly confident, spending impulsively and sometimes hallucinations or delusions. Low mood includes sadness, loss of interest, low energy, sleep and appetite changes, hopelessness, and thoughts of suicide. These moods usually last a few days or weeks, and in between people usually have times when their mood is stable, which can last weeks, months or years.

A GP who suspects bipolar disorder refers to a psychiatrist. The NHS says it can take time to diagnose because it affects everyone differently and symptoms are similar to other mental health conditions. It cannot be cured, but medicines that help stabilise mood and talking therapy can help. The NHS warns not to stop taking bipolar medicine unless told to by a doctor, even if feeling better.

The NHS groups BPD symptoms into four areas: emotional instability, disturbed patterns of thinking or perception, impulsive behaviour and intense but unstable relationships. People may feel rage, sorrow, shame, panic or long-term emptiness and loneliness, with moods that swing in unpredictable ways. Impulses can include self-harm and reckless behaviour such as binge drinking, spending or gambling. Relationships may swing between fear of abandonment and feeling smothered, leading to what the NHS calls a "love-hate" pattern.

The NHS says the causes are unclear but appear to involve genetic and environmental factors, and most people with BPD have experienced some kind of trauma or neglect as children. Many people overcome their symptoms and recover over time. The NHS also notes that the way personality disorders are diagnosed is changing, so that some people may be diagnosed with mild, moderate or severe personality disorder with particular traits instead of a named type.

Both conditions can involve irritability, impulsivity, risky behaviour, sleep disruption, depression and self-harm, and both can be distressing for the person and those close to them. Rethink Mental Illness notes some BPD symptoms are similar to bipolar disorder and complex PTSD. In BPD the NHS even describes brief episodes of unusual experiences, such as hearing voices, and in bipolar disorder mania can include hallucinations and delusions.

The NHS descriptions point to features a specialist can weigh: how long a mood state lasts, whether it comes as distinct high or low episodes with stable periods between, whether the four BPD symptom areas are present, whether the pattern emerged in adolescence and how the person relates to others. A history over time is more informative than a single snapshot. Because the two can coexist, assessment may result in either, both or a different diagnosis altogether.

The NHS advises seeing a GP if you have extreme mood changes that last a long time or affect your everyday life. If you have thoughts of suicide or have harmed yourself, the NHS advises calling your GP or out-of-hours service, dialling 999 if you have taken an overdose or self-harmed, or calling the Samaritans on 116 123.