In tests on human skin samples and mice, the drug tricked the skin into producing the brown form of the pigment melanin, that gives human skin, hair and eyes their colour.
According to researchers at Massachusetts General Hospital in the US, the finding could prevent skin cancer and even slow the ageing process.
Ultraviolet (UV) light makes the skin tan by causing damage. This kicks off a chain of chemical reactions in the skin that ultimately leads to dark melanin - the body's natural sunblock - being made.
"It has a potent darkening effect. Under the microscope it's the real melanin, it really is activating the production of pigment in a UV-independent fashion," David Fisher, one of the researchers, told BBC News.
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It is a markedly different approach to fake tan, which "paints" the skin without the protection from melanin, sun beds, which expose the skin to UV light or pills that claim to boost melanin production but still need UV light.
The scientists want to combine their drug with sun-cream to give maximum protection from solar radiation.
The research was published in the journal Cell Reports.