In March, Robert Berger – a 69-year-old, self-proclaimed “better-living-through-chemistry type of person” – started taking a small dose of rapamycin once a week. The goal was to increase his “health span” – the amount of time he might live without serious disease.

Rapamycin is typically prescribed to organ transplant patients to suppress their immune systems. But many scientists and longevity seekers such as Berger think the drug can do much more than that. They say it can delay ageing and age-related diseases.