William Steere wants to make Pfizer the world's top drugs company by the time he retires in 2001. So far he has helped transform one of the industry's also-rans of a decade ago into a top-ranking pharmaceuticals company, with global sales outstripped only by Merck and Glaxo Wellcome.

But even if he achieves his ambition, he is likely to be remembered for something quite different: Viagra. Pfizer's impotence pill this year became not only the fastest selling new drug in history, following its US launch in April, but also one of the great dinner party topics of our time. "I have been characterised periodically as the chairman of the Viagra company," he notes wryly.

But that's okay. After all, Viagra exemplifies "the ability to transfer knowledge" from the research lab to the market, which is the life-blood of any pharmaceuticals company.

"The science behind Viagra is extraordinary," he enthuses. "The drug was originally developed in our British labs for angina, for increasing blood flow to the heart. They found certain receptor sites that were unique. . . unbeknown to us, there were more of these receptor sites in the penis than in the heart, so our volunteers at Canterbury hospital, as a side effect, were making declarations of erections. There was a certain serendipity behind [the discovery]," he says.

The drug is also notable for the way it has transformed peoples' lives. Mr Steere has been inundated with thank-you letters. "People who have had a disrupted relationship with a spouse of many years say they are now able to resume sexual activity. The letters are just heart-rending."

The only comparable, though less prolific, response was to anti-depressant Zoloft, and a drug for arthritis in dogs. "People spend more money on their companion animals than they spend on their children," observes Mr Steere.

Viagra has also turned the spotlight on the issue of reimbursement. Most Americans pay for health insurance, either privately or through company-wide plans. For health maintenance organisations (HMOs), the companies that provide this health coverage, Viagra is not good news: it means higher costs because it treats a condition that was more or less untreatable. Many have refused to reimburse the price of the pills.

As health care cost constraints mount in the US and Europe, Mr Steere expects reimbursement to become increasingly controversial, particularly for drugs that do not treat a serious or life-threatening condition. Since Pfizer is actively looking for lifestyle and quality-of-life products to sell to ageing populations in developed countries, the issue is not likely to go away.

Some HMOs have refused to reimburse Viagra because they cannot differentiate between men with a real physical problem and those who just want a "lifestyle" drug. Others have limited reimbursement to six or eight pills a month.

Mr Steere believes impotence is a genuine medical problem and is sceptical about the notion of recreational sex: "Ninety nine per cent of sex is recreational anyway. Very little sex is for procreation."

Some HMOs say their reluctance to reimburse Viagra is due to safety concerns. In the US, 69 Viagra patients have died so far, mostly men over 60; more than 3.5 million prescriptions have been written. Mr Steere is annoyed that HMOs will not come clean and admit that their real concern is cost.

"Some people are saying they are not going to reimburse it because of safety issues. In fact it's a bogus excuse, because the issue is reimbursement. Viagra is a very safe drug. The risk is sex. Sex has a very well defined risk associated with it." Viagra labels warn that it is dangerous if used by men taking nitrates, a drug for treating heart problems, but do not spell out that sexual activity itself can be risky.

Still, Viagra has had excellent sales without full reimbursement in the US, says Mr Steere. It will be launched in Europe - initially, at least, without reimbursement in many cases - on October 1st, assuming final clearance is granted.

Perhaps partly as a result of reimbursement problems in the US, after its initially strong sales, new prescriptions for Viagra have fallen off. But repeat prescriptions are going strong, Mr Steere says, and it is still set to become a "blockbuster" drug, with sales of $1 billion in its first year. Mr Steere says the intense initial interest in the product is fading. He says it is no longer the subject of jokes by David Letterman, the late-night chat show host.

He does not regret this. "Even though Viagra is about 5 per cent of our sales it's about 95 per cent of our publicity . . . any discussion of Viagra always kind of drifts from the scientific to the salacious."

But, he points out, Pfizer is far from a one-product company, listing a raft of market-leading drugs. And, he says, "our pipeline is considerable. Analysts consider it the best in the industry". Among products due next year is Celebra, an arthritis treatment to be marketed with Searle, and another potential blockbuster.

So, if not for Viagra, how would Mr Steere like to be remembered? His answer is that of the businessman rather than the scientist: "focus". He has divested about 20 businesses, including medical devices, since taking the reins at Pfizer. He has refashioned it into a pure pharmaceuticals company, active in a broad range of therapeutic areas.

Like other pharmaceuticals companies, Pfizer constantly has to produce new drugs to replace the earnings of old ones that are going off patent. And the company's recent earnings growth of more than 20 per cent is a hard act to maintain.

Pfizer does not face any big patent expiries in the near term - one reason why Mr Steere has every chance of achieving his ambition of leap-frogging Merck and Glaxo Wellcome. Still, "if your pipeline fails you or if your research lets you down, you are basically out of business", he says.

But Mr Steere, like his peers at two other top US drugs companies, Merck and Eli Lilly, says he will eschew the megamergers sought by others in pursuit of growth. "Most [pharmaceuticals] mergers have been out of weakness," he says.

Instead, Pfizer has pursued a strategy of partnership with other big companies in co-marketing drugs, such as cholesterol-lowering Lipitor with Warner-Lambert, and investment in a vast array of small companies.

These days, Mr Steere believes, no single company can hope to pursue every opportunity. "This is the golden age of science," he says. And he appears to be thoroughly enjoying it.