But when an industry thinks it is about to become the latest target of the film maker Michael Moore, precautions have to be taken.

According to the Los Angeles Times, at least six of America's largest pharmaceutical firms have issued internal notices to their workforces warning them to be on the lookout for "a scruffy guy in a baseball cap" who asks too many questions.

Rotund and amiable he may seem, but this could be Moore, digging for dirt for his new movie, provisionally entitled Sicko.

Having watched the Bush administration and the gun lobby come a cropper in Moore's last two works, the pharma giants are not taking any risks.

"We ran a story in our online newspaper saying Moore is embarking on a documentary - and if you see a scruffy guy in a baseball cap, you'll know who it is," Stephen Lederer, a spokesman for Pfizer Global Research and Development, told the LA Times.

Five other big companies have told employees that any approach by Moore should be rebuffed and referred to the company's corporate communications department.

"Moore's past work has been marked by negativity, so we can only assume it won't be a fair and balanced portrayal," said Rachel Bloom, executive director of corporate communications at AstraZeneca, which is based in Delaware. "His movies resemble docudramas more than documentaries."

The US pharmaceutical industry has been criticised in recent months for being undercut by cheaper Canadian equivalents.

Some well-known drugs have been taken off the market after they were shown to have serious side effects.

"We have an image problem - not only with Michael Moore, but with the general public," said MJ Fingland, senior director of communications for the lobbying group Pharmaceutical Research and Manufacturers of America.

Rumours are already swirling within an industry that is becoming paranoid about Moore's movie-making tactics.

Moore, it is said, has hired actors to portray pharmaceutical salesmen who offer gifts to doctors who promote their products.

There is also word that he has offered physicians $50,000 (nearly £26,000) apiece to install secret cameras in their offices in an effort to document alleged corruption.

Ms Bloom said Moore had been spotted at all six of her business centres nationwide. "Michael Moore is becoming an urban legend."