Consumer group Public Citizen, which has urged a ban on prescription Xenical (orlistat), said the new non-prescription diet pill Alli should not have been approved because of pre-cancerous colon lesions recorded during animal studies.

Glaxo officials said those concerns were not valid. Xenical will continue to be sold by prescription, even though in January 2006, a federal advisory panel recommended that the FDA make the weight-loss medication Xenical available without a prescription.

Glaxo previously estimated that 5 million to 6 million Americans would buy Alli over the counter.

There are also some contrainditations for Alli. People who have had an organ transplant should not take this new diet pill because of possible drug interactions, the FDA said.

Patients taking blood-thinning medicines or being treated for diabetes or thyroid disease should consult a doctor before taking it, the agency added.

Even though weight-loss drugs have a role to play in medical treatment, their long-term safety is unknown, says Harvard Women’s Health Watch. And like all medications, they can pose problems. Xenical, which inhibits the body’s ability to take in fats, can also interfere with the absorption of certain vitamins.

The National Institutes of Health advises that diet drugs be taken only by severely overweight people who need them for health reasons, and only in combination with lifestyle modifications.

For example, one study found that people taking the weight-loss drug Meridia (sibutramine) who also received counseling on diet and exercise lost twice as much weight as those who received either Meridia or counseling alone.

In any case, a new weight-loss solution would be welcome since two-thirds of adults in the United States are overweight or obese and at risk for major health problems and early death.

But the reasons for weight problems are complex. No pill can melt away fat or keep the pounds off. Yet for people whose health is at risk, drug therapy may increase the odds of success.