Sinclair smiled. ‘Psychosis is only a symptom of mental illness, Detective Constable. The subsequent diagnosis was bipolar disorder. Mr Lowther was treated with anti-psychotic drugs until the episodes receded, and then we adjusted his medication until he was stable enough to be discharged.’

‘So the medication made the voices go away?’

‘Well . . .’

Cooper watched him hesitate.

‘But only as long as he kept taking the tablets?’ he said.

‘Of course. Anti-psychotic drugs work by changing the activity of chemicals that transmit messages in the brain. It’s very important to take the medication regularly, and at the prescribed doses.’

‘So when was John Lowther discharged from the unit?’

‘In April. He was in good spirits by then. He said he was going to tell everyone that he’d been away on holiday. Somewhere nice, like the Caribbean. Except that it couldn’t be the Caribbean, because he didn’t have a sun tan. I dare say his family didn’t let anyone know the truth either. That’s perfectly usual. There’s still a lot of stigma attached to mental illness, I’m afraid.’

‘He’d have to be on anti-psychotic drugs for some time, I imagine.’

‘Yes. But his records do show that he was worried about the side effects. Mr Lowther complained that he put on weight. And he twitched a lot, which he found distressing. Also, he said the medication made him impotent.’

‘He was very restless when we saw him last week.’

‘But no twitching?’

‘No.’

Sinclair fiddled with his glasses. ‘Mr Lowther was prescribed Orphenadrine for the side effects, but he didn’t like the idea of taking more tablets. So instead . . .’ He hesitated again.

‘You think he might have stopped taking his anti-psychotic drugs?’

‘Yes, it’s possible. Dealing with side effects is always a bit of a trade-off. It’s a question of striking the right balance for each individual. Mr Lowther said once or twice that the effects of the drugs were worse than a few harmless delusions. He thought he’d return to his old self if he stopped taking the medication – he’d become slimmer, stop twitching, regain sexual function. It’s a common response. Patients feel an overwhelming desire to go back to a time when they felt happy and safe.’

‘Hold on a minute – “a few harmless delusions”?’ said Cooper, picking out the most remarkable phrase in what he’d just heard.

Sinclair fixed him with a sad gaze from his pale eyes. ‘That would be the way it seemed to him, at this particular stage. I think Mr Lowther had probably forgotten the nature of his auditory hallucinations.’

‘How is it possible to forget something like that?’

‘It’s a normal function of the brain to filter out negative experiences and retain the positive ones. This applies to psychotic episodes as much as to anything else.’

‘I can’t see anything positive in what you’ve described, sir.’