Two herbs used as folk therapy contraceptives contain chemicals that block a key step in fertilisation, a study has found.
Extracts from the plants could provide the basis of a new kind of non-hormonal "molecular condom" with fewer side effects than the Pill, scientists believe.
In the folk medicine tradition, thunder god vine and aloe vera are both claimed to prevent unwanted pregnancies, as well as having other beneficial effects.
Scientists identified two molecules - lupeol from aloe vera and pristimerin from thunder god vine - that in laboratory tests prevented sperm breaking through a protective "wall" of cells surrounding the egg.
Affected sperm were unable to generate the final "kick" from their whip-like tails that forces them through the barrier.
Fertilisation cannot occur unless sperm overcome this obstacle.
The chemicals could serve as an emergency contraceptive taken before or after sexual intercourse, or as a permanent method of birth control via a skin patch or vaginal ring, said the researchers.
US lead scientist Dr Polina Lishko, from the University of California at Berkeley, said: "Because these two plant compounds block fertilisation at very, very low concentrations - about 10 times lower than levels of levonorgestrel (an emergency contgraceptive) - they could be a new generation of emergency contraceptive we nicknamed 'molecular condoms'.
"If one can use a plant-derived, non-toxic, non-hormonal compound in lesser concentration to prevent fertilisation in the first place, it could potentially be a better option."
The key to the sperm "power kick" is the opening of a molecular gateway that allows an influx of calcium to the cell's tail, said the researchers writing in the journal Proceedings of the National Academy of Sciences.
Last year, the same team found the hormone progesterone plays a key role in opening the "calcium channel" and triggering sperm tail-whipping.
Certain hormones were found to block the channel, including the male hormone testosterone and stress hormone cortisol.
A search for non-hormonal chemicals that might have the same effect brought to light natural contraceptives used by indigenous people around the world.
One was pristimerin, from the plant Tripterygium wilfordii, also known as "thunder god vine".
Leaves from the plant are used both as an anti-fertility drug and as a remedy for rheumatoid arthritis in Chinese traditional medicine.
The other chemical was lupeol, found in a number of plants including aloe vera, mango and dandelion root.
Aloe vera gel mixed with lemon juice has traditionally been used as a spermicide.
Both pristimerin and lupeol stopped progesterone opening the vital calcium channel, the scientists discovered.
Co-author Dr Nadja Mannowetz, also from the University of California at Berkeley, said: "These compounds not only blocked calcium channel activation, but also blocked sperm hyperactivated motility, reducing their activity to the level of non-activated sperm cells.
"It doesn't kill sperm basal motility. It is not toxic to sperm cells; they still can move.
"But they cannot develop this powerful stroke because this whole activation pathway is shut down."