It is the biggest and most powerful drug company on the planet. Its state-of-the-art glass headquarters in Surrey and vast research park in Sandwich, just north of the old Cinque port, boast of the £1bn it has put into the UK economy since 1996. Its famous blue diamond-shaped Viagra pills have made it a fortune beyond the dreams of small nation states and the butt of smutty jokes worldwide. But Pfizer's global reach has not turned the world's third largest business into a benevolent giant, according to its critics. Quite the contrary. The vast multinational stands accused of blocking reforms to global drug pricing that would help lift impoverished countries out of disease and spur their development.

As shareholders gather for the annual general meeting in Michigan this morning, their investments snugly coining profits at the rate of $1m an hour, critics are asking increasingly urgent questions about Pfizer's attitude to those who need medicines in poor countries. The takeover last week of Pfizer with Pharmacia, which took the New York-based giant past its British rival GlaxoSmithKline to become the largest company of the most profitable industry on the planet, makes it all the more important, they say, that Pfizer takes a lead in conceding a system of low prices in the 90 per cent of the world worst hit by disease.

The battle for low prices for the poor pits the basic instinct of huge American corporations to make money against a growing public belief that pharmaceutical companies have a special humanitarian duty to help the sick, wherever they are in the world and however little they can afford to pay. Already the major drug companies have slashed the prices of Aids drugs, albeit as a result of a public outcry combined with competition from generic companies making cheap copies of their drugs in countries that do not yet acknowledge their patents.

But now the stakes have risen higher still. The drug companies have recognised that they must be seen to help in the Aids crisis, but Pfizer and the rest want to draw a line in the sand, separating Aids, malaria and tuberculosis and similar infectious pandemics from the insidious and profitable diseases that affect millions in rich countries as well as poor, such as cancer. Cheap Aids drugs - yes. Cheap cancer drugs? Well, says Pfizer, poor countries haven't asked for them. They've got far too many other problems.

"I've got to tell you, I don't know very many health ministers in very many poor countries that are approaching pharmaceutical companies, generic or non-generic, asking for cancer drugs," says Robert Mallett, Pfizer's combative vice-president for corporate affairs.

But Rafael Bengoa, director of management of non-communicable disease at the World Health Organisation, says they had better look out. The struggle over Aids drugs will inevitably become a struggle over insulin and cancer drugs too. While Aids and Sars hit headlines, he warns, "the invisible epidemics are killing more people than those".

Pfizer says talk of cancer drugs is irrelevant. This is a massive but in many ways old-style American corporation, making megabuck profits that to US eyes look more impressive the higher they climb. For over 150 years, since young Charles Pfizer left Germany with $2,500 borrowed from his father and set up in Brooklyn with his cousin Charles Erhart, the company has prided itself on its entrepreneurial spirit and the quality of its products. Their first big hit was santonin, a bitter and unpopular medicine used to treat intestinal worms. The pair, inspired by Erhart's confectionery background, mixed it with almond-toffee flavouring, moulded it into a cone shape, and never looked back. A smart line in painkillers, preservatives and disinfectants during the civil war confirmed Pfizer as a runaway success story. In 1955, it began selling antibiotics in the UK. By 1972 it had crossed the billion-dollar sales threshold. By 1980, it had its first billion-dollar single drug - the anti-inflammatory medicine piroxicam, brand-name Feldene.

It's a classic American success story. Pfizer has made its own luck, called all the shots and has now reached the top of the pinnacle. But the pressure is on to introduce the kind of tiered pricing system - different rates for different states - that runs counter to its philosophy.

Campaigners who took to the streets and lambasted the drug giants over the appalling spectacle of young men, women and children in Africa dying of Aids, which is treatable although not curable in rich countries, have now widened the grounds for complaint. They want a transparent system which will allow poor countries to waive the multinational giants' patents and buy or make whatever cheap drugs they need to safeguard the health of their people. That means cancer, asthma, diabetes as well as Aids, TB and malaria.

In Geneva, negotiations