Pfizer's move comes amid its American parent's reiteration of guidelines for appropriate use of Viagra, "as well as caution against inappropriate use".

About 1 million Viagra prescriptions have been written out since its launch early last month.

The company has stressed that patients taking nitrates in any form, including nitroglycerin and long-acting nitrates commonly used for chest pain, should not take Viagra.

"Large and sudden drop of blood pressure can occur with the co-administartion of these two drugs," a Pfizer Inc communique says. The letter is, however, part of the normal process of educating doctors about side-effects of new drugs.

While Pfizer's official spokesman was unavailable for comment on the matter, sources say the company was expected topress against indiscriminate grant of approvals to manufacture and sell sildenafil citrate both within the country and abroad.

Earlier last week, the multinational, while maintaining that the physical molecule was not difficult to synthesise, said that it possessed the `sum of cumulative experiences' with Viagra.

It added that the reams of information made available at the time of seeking patents was meant to further the frontiers of science.

Analysts, however, said that domestic firms which had made applications to the Drug Controller of India had not violated any law, and there could be no specific reason to deny them a licence.

"Sildenafil citrate is just like any other new bulk active, and most Indian companies targeting export markets are backed by sound process-research capabilities and also have their own reputation at stake.

Moreover, all domestic launches will be backed by the requisite clinical trials. Denying them permission would only mean significant forex losses," an analyst said.Sildenfil citrate, experts add, currently commands a price of around $16,000 to $23,000 per kg. Industry experts say that denying a clearance may also have larger implications on the exclusive marketing rights front under the WTO, once an ordinance is passed here. The "black box" clauses give a patent-holder five years of exclusive marketing rights in India for completely new drugs "which are not in public domain" at the operative date and for which marketing approval is granted in India before the end of the transitional 10-year period.