One day in the early 1980s a distressed patient went to see the doctor Heather Ashton at her pharmacology clinic in the Royal Victoria hospital, Newcastle upon Tyne.

The patient had been given the tranquilliser Ativan (lorazepam) to relax her muscles before an operation and now feared she was addicted to the drug. Ashton listened to her patient and many others like her, studied their problems scientifically, and concluded that addiction to benzodiazepine tranquillisers was a serious problem, associated with severe withdrawal symptoms. The manual she went on to write on how to quit benzodiazepines safely now provides the basis for practice all over the world.

Benzodiazepines, including Valium (diazepam) and temazepam, became popular in the 60s as apparently safe and effective drugs to treat anxiety or insomnia. They quickly became some of the biggest earners for the pharmaceutical industry.

In 1984, Ashton, who has died aged 90, started a unique clinic for people who had become addicted to tranquillisers. She recognised that services designed for people who abused illegal substances were inappropriate for people whose suffering was caused by medication they had taken, sometimes for years, on the advice of their family doctors.

Ashton accepted that benzodiazepines were useful, but argued that they should be prescribed for no more than two to four weeks. Beginning in the mid-80s she rigorously collected data and published papers on the adverse effects of long-term use, and the problems of withdrawal.

She developed an approach to withdrawal that supported the patient to control the rate at which the dose was tapered, often taking months or even longer. In 1999 she distilled her experience into a manual, Benzodiazepines: How They Work and How to Withdraw. There was such demand that the book, known worldwide as the Ashton Manual has since appeared in 11 languages and several updated editions, all available to download for free.

Her views were initially challenged by some psychiatrists. However, by the late 90s most accepted that long-term use of benzodiazepines was not safe. In 2013 the British National Formulary, following an appeal from the British Medical Association, revised its guidelines on withdrawal to align with the latest edition of the Ashton Manual. Millions of patients worldwide have benefited from these changes in practice.

Born in Dehradun, northern India, Heather was the daughter of Harry Champion, a silviculturist at the Imperial Forest Research Institute, and his wife, Chrystal (nee Parsons), who had been his boss’s secretary.

Heather, the younger of their two children, was left largely in the care of nursemaids. At the age of six she was sent to Oldfeld school in Swanage, Dorset, a “progressive”, though spartan, mixed boarding school. She remembered it as a miserable period of her life.

At the outbreak of the second world war in 1939 she was dispatched across the Atlantic in a convoy that was attacked by U-boats, evacuated to live with a family in West Chester, Pennsylvania. They became a second family to her and she was able to develop her passionate curiosity about the world at Swarthmore high school.

In 1945 she was reunited with her parents in Oxford, where her father had become director of the Imperial Forest Research Institute, and she studied medicine at Somerville College. She fell in love with John Ashton, an ex-naval airman and the son of a jobbing gardener from Liverpool, who was studying agricultural economics.

They overcame her parents’ class-based objections, married, and went to live in London, where he worked as an economist for the Ministry of Agriculture. She completed her professional training at the Middlesex hospital, undertook research projects in cardiovascular medicine and gave birth to the first three of their four children.

In 1964 John was appointed professor of agricultural economics at Newcastle University. The family moved to Jesmond and their fourth child was born in 1966. Heather joined the department of pharmacological sciences at the university, which had recently been established by John W Thompson.

Under his influence she developed her expertise in the effects of psychoactive drugs, and adopted new research methods such as electroencephalography. Ashton was also becoming an inspiring teacher of medical students, continuing to combine curiosity and compassion in her practice as a doctor.

Her research also included the effects on the brain of substances including nicotine and cannabis, and she undertook studies of drinking and drug-taking among medical students. After retiring from her personal chair in clinical psychopharmacology at Newcastle, she continued to be active on the executive committee of the North East Council on Addiction and to lecture – unpaid – to medical students.

Inexhaustibly energetic, she also took the opportunity to pursue her love of travel, the outdoors and sport – she had been a competent squash player and continued to enjoy tennis and badminton. Her health deteriorated in 2015, but, while cared for by her family at home, she continued assiduously to reply to emails from all over the world from people desperate for advice on the side-effects of their medication.

She avoided the limelight and was reluctant to be drawn into any kind of confrontation. However, she was uncompromising in the evidence she gave to parliamentary committees and in her conference presentations, being particularly scathing in her assessment of drug companies and their overriding focus on profit.

Her husband died in 1986. She is survived by her children, John, Caroline, James and Andrew, and six grandchildren.

• Chrystal Heather Ashton, psychopharmacologist, born 11 July 1929; died 15 September 2019