To find out if adding quetiapine to treatment as usual (TAU) can improve the mental health of individuals with Borderline Personality Disorder (BPD) in comparison to a dummy treatment (placebo) and treatment as usual. The placebo and quetiapine will look, taste, smell and feel the same so that participants do not know which one they are receiving. This is important so we can be sure any differences in outcomes between the two groups are only due to the treatment received.

Borderline personality disorder (BPD) describes a collection of problems including negative feelings about self, fears of being let down, an acute sense of abandonment and rapid, distressing changes in mood. People with BPD find it difficult to maintain relationships, have high levels of mental health problems like depression and drug misuse, and high rates of self-harm and suicide.

Quetiapine is the most widely prescribed antipsychotic medication for BPD in the UK despite limited evidence that it works. There is only one published trial of quetiapine for BPD treatment which found that quetiapine was more effective than a 'dummy tablet' (placebo) in improving symptoms of BPD. However, the trial was short with a small number of participants so a longer and larger trial of quetiapine is required to see if these promising results can be repeated. It would be a very important finding and provide evidence for use of quetiapine to treat BPD. If the result does not provide evidence of the benefits of quetiapine, then clinicians will review whether treatment with quetiapine should be continued.

In this trial, people with BPD will be allocated, by chance, to receive either placebo or quetiapine for 12-months alongside their usual treatment. We will recruit people in contact with mental health services in the NHS in regions in England that are under-represented in mental health research.

We will also examine any changes in cost that result from prescribing this drug and whether any changes in costs are worthwhile in terms of improvements in outcomes.