Shilajit is the latest supplement marketed online as a “natural testosterone booster”. Promoted by influencers, wellness brands and biohacking communities, this sticky black resin is advertised as a way to increase testosterone, improve energy and enhance male vitality.

Shilajit is a tar-like substance formed over centuries from decomposed plant material compressed within rock in high-altitude regions such as the Himalayas. It has a long history of use in traditional Ayurvedic medicine, where it has been described as a rejuvenator and as an anti-aging compound.

Chemically, it contains a mixture of fulvic acid and trace minerals. Its complex composition is part of its mystique and also part of the scientific challenge in studying it.

When it comes to testosterone specifically, the evidence for shilajit’s effect on testosterone levels in humans is limited to two small studies.

The most frequently cited study was published in 2016. In this trial of healthy men aged 45–55, half were given purified shilajit (250mg twice daily) and half were given a placebo for 90 days. Neither group knew which group they had been randomly assigned to. The shilajit group saw increases in both total and free testosterone – the form immediately available for the body to use – compared with the placebo group.

On the surface, this sounds encouraging. However, the study included just 38 participants treated with shilajit, was of a relatively short duration, and measured testosterone using methods that are not considered the most reliable.

Although testosterone levels were modestly higher at 90 days, measurements at 30 and 60 days fluctuated in both directions. And probably the most curious finding was that testosterone levels fell in the placebo........

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