The Office for National Statistics has surveyed the fatal toxicity of all antidepressants over 10 years, based on data in coroners' reports in England and Wales.

The government agency's survey found that deaths associated with venlafaxine, a drug sold in the UK under the brandname Efexor by Wyeth, a US pharmaceutical giant, were equivalent to 8.5 for every million prescriptions since its launch in 1995.

The drug, known as a serotonin and noradrenaline re-uptake inhibitor (SNRI), is more powerful than the family of drugs including Prozac and Seroxat that are known as selective serotonin re-uptake inhibitor (SSRI). It is most commonly prescribed for psychiatric inpatients.

The agency said deaths associated with Efexor were a lot more than the SSRIs, which averaged one death for every million, but less than the tricyclic family of antidepressants including dothiepin and amitriptyline, which averaged 43.1 for every million.

Its concern was based on annual figures showing deaths associated with Efexor are on a rising trend, while the figures for rival drugs are falling. "You could interpret this as a warning signal," an official said. An article in the ONS publication Health Statistics Quarterly said: "Encouragingly, for SSRIs there has been little change in mortality per million prescription items, despite the very large increase in their usage." It associated the higher mortality figures for Efexor with its greater toxicity.

Despite this apparent danger, it is understood that the National Institute of Clinical Excellence is preparing to give its seal of approval to Efexor. Paradoxically, the Medicines and Healthcare Products Regulatory Agency is close to completing a review of the safety and efficacy of SSRIs, including the bestselling Seroxat, and is expected to impose restrictions on their use.

Since 1995, Efexor has built up an annual turnover of £125m. It sells about 2m prescriptions a year, and is gaining a rising share of the total market of 26.3m prescriptions.

A spokeswoman for Wyeth said: "These figures of deaths per million do not take into account the severity of the condition of people being treated, or the fact that patients being given Efexor have often failed on other antidepressants.

"Patients who are severely ill are being treated on this drug and that is why the figures are higher."

Celia Feetam, the immediate past chairman of the UK Psychiatric Pharmacy Group, said: "These statistics are not surprising. Venlafaxine is an effective antidepressant, and, as such, its use is likely to increase. However, there are effective alternatives available for the treatment of mild to moderate depression, which are safer in overdose and which should be used in preference as firstline treatment.

"It is important to acknowledge the toxicity of venlafaxine in overdose, and it should be used only with caution in those patients who may be at risk of self-harming and who are not under specialist care."

Sophie Corlett, director of policy at Mind, the mental health charity, said: "Efexor is a relatively new antidepressant, and so reports of adverse side-effects have only recently begun to emerge. Mind has been aware for some time of reports from mental health service users of quite debilitating side-effects, ranging from dizziness and nausea to severe headaches, and dependency problems. Although on the whole Efexor has a better reputation than other antidepressants, in a recent survey almost half of respondents reported difficulties when coming off the drug or in reducing its dose.

"It is important to remember that the effectiveness of antidepressants can vary enormously from individual to individual - some will take Efexor and feel that it has made an important positive difference to their lives.

"However that is certainly not always the case, and questions still remain over the links between certain pharmaceuticals and possible increased suicidal feelings.

"It is therefore vital both that alternative non-drug therapies are explored for less serious cases of depression, and that, where drug treatment is the only realistic option, those drugs are rigorously tested and regulated.