It is illegal for a drug company to promote a medicine for uses not approved by the government, though it is not illegal for doctors to prescribe medicines for so-called off-label uses.

It is illegal for a drug company to promote a medicine for uses not approved by the government, though it is not illegal for doctors to prescribe medicines for so-called off-label uses.

Warner-Lambert's shadowing program involved an estimated 75 to 100 doctors in several Northeast states, Dr. Franklin estimated in court documents. Each doctor was paid $350 or more for each day they let sales representatives watch as they examined patients, according to court documents.

Other companies also pay doctors to open their doors to sales people.

The federal investigation, which stems from the whistle-blower lawsuit, centers on marketing activities that took place in the mid- to late-1990's, before Pfizer bought Warner-Lambert in 2000. The lawsuit argues that Medicaid paid tens of millions of dollars it should not have for Neurontin prescriptions written for unapproved uses.

Pfizer said that in 2000 more than 78 percent of Neurontin prescriptions were written for unapproved uses. Sales of the drug are growing at a rate of 50 percent a year - fueled mostly by those off-label uses.

Neurontin has been approved by the F.D.A. for a very narrow use: controlling seizures in epileptics who already take another drug. But one marketing executive at Warner-Lambert, in a recorded voice-mail message that is part of the lawsuit, told sales representatives: "If we are going to market Neurontin effectively, we have to do it for monotherapy, for epilepsy, also for pain and bipolar and other psychiatric uses." (Monotherapy refers to using a single drug to treat a condition, which is not an approved use of Neurontin.)