A new study has revealed a concerning link between using marijuana and pregnancy. Scientists say tetrahydrocannabinol (THC), the ingredient that triggers marijuana’s psychological effects, may also lead to infertility in women.

According to a study on embryos at the University of Guelph in Canada, exposure to THC leads to a drastic drop in the chances of a successful pregnancy. Testing the eggs of cows, researchers found that higher levels of the marijuana ingredient delayed the eggs from reaching important milestones in their development.

Megan Misner, one of the study’s authors, says doctors regularly warn women to avoid marijuana while getting fertility treatments. Until now however, Misner says the evidence to support that advise hasn’t been strong.

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Legalising cannabis would result in soaring numbers of people suffering from schizophrenia-like psychosis, one of Britain’s top psychiatrists has warned.

Evidence now shows that when the drug is legalised, greater numbers smoke it more frequently and in stronger varieties.

These factors increase the incidence of cannabis-related psychosis, according to Professor Sir Robin Murray, an authority on the risks of the drug to mental health.

He predicted that ‘big cannabis’ firms with scant regard for people’s health will ‘seduce’ the Government into reforming the law – and the State will then find itself in thrall to the new industry because of the tax and jobs it provides.

The warning is a major intervention from Professor Murray, who told The Mail on Sunday growing evidence about the harm cannabis causes had made him change his mind about legalising the Class B drug.

Previously the expert – a professor at the Institute of Psychiatry at King’s College London – had supported tightly controlled legalisation married with public education campaigns spelling out the risks.

But he said the experience of places that had decriminalised or legalised it – from Portugal and the Netherlands to swathes of North America – had made him think again.

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One US study they cited found 30 per cent of users were now dependent, triple the proportion in the 1990s, with dependency growing as the strength of the drug increased. That finding accords with research published last week which found the drug can be highly addictive and can cause serious withdrawal symptoms.

Some reports found cannabis use increases the risk of depression and suicide, said the pair, ‘but by far the strongest evidence concerns psychosis’.

‘Numerous prospective studies have shown cannabis use carries an increased risk of later schizophrenia-like psychosis,’ they warned.

In fact, ten of 13 such studies showed users had ‘a significantly increased risk of psychosis’ while ‘two of the remaining three showed a trend in that direction’. Those who smoked ‘high-potency cannabis’ daily saw their risk of psychosis increase ‘up to nine-fold’.

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