A new drug promises to spare hundreds of thousands of Alzheimer’s sufferers from “terrifying” hallucinations and paranoia, researchers have announced.

Scientists have successfully tested the first medicine capable of treating psychosis, which affects around half of patients with the disease, without the devastating side-effects caused by current drugs.

Published in the Lancet Neurology, the trial of pimavanserin offers particular hope to those with advanced psychosis, which doctors often describe as the most distressing symptom of Alzheimer’s.

Currently people with dementia rely on mainstream antipsychotics, however these can double the speed at which brain function deteriorates, increase the risk of falls and lead to sedation.

Figures indicate they also cause 1,660 unnecessary strokes and 1,800 unnecessary deaths in the UK every year.

By contrast, pimavanserin, which targets a specific nerve receptor in the brain, was shown to significantly improve psychosis symptoms without the normal side-effects.

Researchers at the University of Exeter Medical School tested the drug on 90 patients with Alzheimer’s disease, giving a placebo to a further 90.