One in five people in the UK is obese
The drug, also known as sibutramine, has been linked to two deaths and more than 200 reported adverse reactions in the UK.
Similar concerns have been raised in Italy, where it is banned after two people died, and in France.
But a spokesman for the Department of Health said the deaths were more likely to be caused by specific medical conditions suffered by the individuals.
Out of more than 200 reactions, 93 were deemed serious, including two fatalities.
The spokesman said: "As with all new medicines, the Medicines Control Agency (MCA) and Committee on Safety of Medicines (CSM) are intensively monitoring the safety of sibutramine.
"Patients currently treated with sibutramine (Reductil) can continue to take their medicines as usual.
"However, if they are unwell or concerned they should speak to their doctor."
Side effects
A Europe-wide review of Reductil by the European Medicines Evaluation Agency is looming.
And in preparation, national agencies are studying the number of adverse reactions.
In Italy, two deaths and reports of other health problems prompted sales of drugs to be suspended last week.
In France, drug regulators have recorded 99 reports of side effects, ten of them serious, but no fatalities in patients taking sibutramine.
The company behind the marketing of the drug, Abbott Laboratories, maintains the drug is safe.
It said that 8.6m people have used sibutramine-based drugs since 1997 and there was no change to the risks.
The drug was approved for NHS use by certain groups of patients by the National Institute for Clinical Excellence (NICE) last October.
Insomnia
It works by suppressing the appetite and increasing the speed at which the body feels full.
The rate of weight loss is on average 8% after two years.
But there are potential side effects of insomnia, constipation and a dry mouth.
One in five people in the UK is obese.