Regulators in the USA say that a safety review of Merck & Co’s Singulair (and two other asthma drugs) do not suggest that the blockbuster is associated with suicide or suicidial behaviour but its probe will continue to see if the drug causes mood and behavioural adverse events.

The US Food and Drug Administration has given an update on the review it began nine months ago to look at neuropsychiatric events linked to Singulair (montelukast), as well as two other drugs which act through the leukotriene pathway – AstraZeneca's Accolate (zafirlukast) and Cornerstone Therapeutics' Zyflo (zileuton).

Merck submitted results from 41 trials in patients six years of age and older, of which 9,929 were treated with Singulair and 7780 were treated with a placebo. One adult on montelukast had suicidal ideation and there were no completed suicides and none of the patients in the placebo group reported either of the effects.

Astra Zeneca submitted results from 45 trials in patients aged five and upwards, of which 7,540 were treated with Accolate and 4,659 were on placebo. No-one treated with zafirlukast had suicidal ideation or completed suicide but two in the placebo group had suicidality. Cornerstone filed results from 11 trials in patients aged 12 and older (1745 on zileuton and 1063 on placebo) and no patients reported any problem.

However while the FDA acknowledged that these data do not suggest that the three drugs are associated with suicide or suicidal behaviour, it said that “these clinical trials were not designed specifically to examine neuropsychiatric events” and as a result, “some events may not have been reported”.

Therefore the agency said it has “not yet reached a definitive conclusion” and it will continue the review, “which may take months to complete”.

Merck responded by saying that it “stands by the proven efficacy and safety of Singulair, a medicine that has been prescribed to tens of millions of patients with asthma and allergic rhinitis for more than 10 years”. The firm added that after a thorough review of the clinical trial data “and a careful assessment of post-marketing adverse events, Merck believes that the data support the continued use of Singulair in appropriate patients”.

However in a research note, Sanford Bernstein analyst Tim Anderson wrote that if the FDA safety review “leads to a stern warning about behavioural changes in the Singulair label, this could frighten users of the drug or their parents and give Merck's competitors ammunition to attack the brand". The drug is Merck’s best-selling product and had third-quarter sales of $1.1 billion, down 1%.