If worms could smile, the microscopic Methuselahs in a Seattle lab might be beaming; they're living longer than nature would normally allow.

Research published in the journal Nature on Tuesday showed several antidepressants used in humans can extend the lifespan of a tiny roundworm by about a third.

What's more, the drugs appear to extend their lifespan using the same mechanisms by which caloric restriction extends lifespan in a number of organisms.

The roundworm C. elegans is a favourite of researchers studying lifespan because it's easy to work with and much of its biology — genes, enzymes, proteins — is similar to that in humans.

Linda Buck, the Nobel-prize winning researcher at the Fred Hutchinson Cancer Research Centre in Seattle, said she and her colleagues were curious. Might there be human drugs that would make the worms live longer?

So they screened 88,000 different chemicals and found 115 that increased lifespan by up to 40 per cent.

Of those, one compound was very similar in structure to certain antidepressants that act on the signalling pathways of the neurotransmitter serotonin.

When they tested those drugs on the worm, Buck said they "found four drugs that increased lifespan in C. elegans, and all of them acted on a particular class of serotonin receptors."

The others had little or negative effect, but among the ones that worked, mianserin (Tolvan) is sold in Europe, and another, mirtazapine (Remeron), is marketed all over the world, including Canada. Much of the lab's work focused on mianserin.

Previous studies have shown that restricting calories can make everything from worms to fruit flies to rodents live up to 40 per cent longer. So Buck found it intriguing that the drug uses the same mechanisms as calorie restriction to extend life.

While the creature using mianserin eats normally, Buck thinks its body may perceive it is starving, thus perhaps jolting the life-extending outcome of caloric restriction into action.

While the results are encouraging, Buck cautioned that what works for a roundworm may not carry over to more complex life forms.

"We have no evidence — and this is important — that the drug we found that would increase lifespan in the worm would also do that in mammals," said Buck.

But the researchers intend to find out. They're gearing up to test it in mice.

Buck also said she would like to get information from those longitudinal studies where large populations are followed for many years, just in case enough people have taken one of the drugs for the data to show an effect on their aging — and their lifespans.