The Food and Drug Administration said yesterday that it will take the first step next week toward a possible ban on sales of phenformin, a controversial drug prescribed for about 250,000 persons with mild diabetes.

A spokesman said the agency will publish an official notice in the Federal Register, probably on Friday, announcing the FDA's intention to withdraw its approval for the marketing of phenformin and providing manufacturers and other an opportunity for a hearing.

Giba-Geigy Corp. sells the drug as DBI; USV Pharmaceutical Corp. sells it as Meltrol. Phenformin, which is intended to hold down blood-sugar levels in persons with so-called adult onset diabetes, causes lactic acidosis, a frequently fatal adverse reaction, in some users.

The spokesman said the hearing will center on this issue: whether there is an identifiable population, now unknown to the FDA, that needs the pills. If so, the question then would be whether continuing to make the drug available would entail benefits that outweigh the risks.

The hearings - which has been in the works for several months - would set in motion administration and possibly court challenges that could last for years. Sales could continue in the interim. At retail, volume has been running at an annual rate of $25 million.