It used to be said that fashions took a decade to reach Britain from America. If only, AstraZeneca and GlaxoSmithKline chiefs may be muttering.

For less than two months after Merck stock plunged by one-quarter on fears for the safety of Vioxx, an arthritis drug, the market has turned on AstraZeneca and GSK over concerns about their portfolios. The stock market value of both companies lost more than £2.5 billion within seconds of London trade opening this morning.

The speed with which pharmo-scepticism has crossed the Atlantic reflects in part a growing enthusiasm in the US for tackling perceived slights by bodies based abroad. Rather than waiting for international consensus on tackling abuses, Eliot Spitzer, the New York Attorney General, for instance, has unilaterally targeted widespread, yet unseemly, insurance industry practices on grounds that they are largely taking place on his turf.

The concerns surrounding the UK's two largest drugs companies were prompted by comments from an adviser to the US Food and Drug Administration over the safety of two of their drugs, among "five on the market today that need to be looked at quite seriously to see if they belong there".

The trouble for the companies was that the comments came not from any adviser, but from Dr David Graham, who led the trial which prompted Merck to withdraw Vioxx. The investigation has given him iconic status and made his name a brand of the searing type when applied to further drugs. An examination from Dr Graham has become a fast way for companies to discover who their friends are.

GSK and AstraZeneca's discovered this morning that their chums included analysts at Seymour Pierce, who dismissed Dr Graham as a "bit of a loose cannon". Merrill Lynch noted that GSK relied for only a small part of its sales on Serevent, the asthma treatment which has attracted Dr Graham's attention.

Yet the impact on sales of a drug's withdrawal is not the main issue - it is the loss to lawsuits which are bound to follow. By the end of October, Merck had been cited in 375 Vioxx lawsuits involving some 100 groups of plaintiffs. That might just represent the tip of the claims iceberg. According to one medical journal, "tens of thousands" of Vioxx patients may have suffered "adverse events".

In some respects, pharmaceuticals companies have only themselves to blame for potential losses to US claims. In targeting the lucrative US market – Merck earned $2.5 billion from Vioxx alone last year – drugs makers knew they were entering litigation-happy territory. They knew they would garner the attentions of a new range of regulators.

The trouble comes when operational risks outweigh the potential rewards. When drugs – even those approved by regulators – represent so much of a long-term risk through as-yet undiscovered side effects that companies have second thoughts in introducing them.

Regulation is getting so tough that, in protecting sufferers from potentially harmful drugs, it is delaying the release of prospective life-saving treatments. We hear much about the patients harmed by pills but little of those sentenced to prolonged suffering by delays to a drug's release.

Better educated Western populations should realise that any drug has a side effect. Even Aspirin, lauded for its ability to prevent heart attacks and strokes, can cause fatal internal bleeding.

What should be worrying for Britons is that America's ambulance-chasing litigious culture is crossing the Atlantic, too. You must have seen the "Had an accident at work?" television advertisements.

It is little susprise that pharmaceutical shares, once prized as defensive stocks, are increasingly having a place only in higher risk portfolios.

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