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Two compounds normally found in wild plants could make good alternatives to emergency contraceptives - if scientists only knew where to get enough of them.

Chemicals from dandelion root and the "thunder god vine" plant have long been used in traditional medicines.

Now, Californian researchers have found they can also block fertilisation.

A UK sperm expert said the discovery could lead to a new and novel approach to male contraception.

But the compounds existed at such low levels in plants that the cost of extraction was very high, the US team said.

In tests, chemicals called pristimerin and lupeol stopped fertilisation by preventing human sperm from whipping its tail and propelling itself towards and into the woman's egg.

The chemicals were acting like "molecular condoms", the study authors wrote in the journal .

In other words, they successfully blocked progesterone - which triggers the sperm's forceful swimming - but didn't damage the sperm.

"It doesn't kill sperm basal motility. It is not toxic to sperm cells; they still can move," said Polina Lishko, assistant professor of molecular and cell biology from the University of California, Berkeley.

"But they cannot develop this powerful stroke, because this whole activation pathway is shut down."

Lupeol is found in plants such as mango, dandelion root and aloe vera, while pristimerin is from the tripterygium wilfordii plant (also known as "thunder god vine") and is used in traditional Chinese medicine.

The researchers found that the chemicals worked at very low doses and had no side-effects either, unlike hormone-based contraceptives.

They concluded that the compounds could potentially be used as an emergency contraceptive, before or after intercourse, or as a permanent contraceptive via a skin patch or vaginal ring.

Prof Lishko and her colleagues are now going to test how well these chemicals work in primates, whose sperm cells work in a similar way to humans.

They also are searching for a cheap source of the chemicals, which are very expensive to extract from wild plants because they are present at very low levels.

Allan Pacey, professor of andrology at the University of Sheffield said there was a real need for a non-hormone base male contraceptive.

"This is a very interesting study which shows that two natural compounds can knock out a key molecule on sperm that regulates how they swim in the final moments before fertilisation.

"Moreover, because the molecule is specific to sperm, it seems a good bet that this could be a novel contraceptive target that might lead to a male contraceptive pill without any of the side-effects so far seen in trials with hormone-jab contraceptives."

However, he said clinical trials were needed to show whether it worked in real people and this was likely to take a few years.