How so? Variations in a pair of genes my increase the chance of reporting suicidal thoughts after taking Celexa, according to a study in the latest issue of The American Journal of Psychiatry. (Here’s the abstract). Although preliminary, the results may help researchers develop tests to predict who will react badly to the med, and who will do well.

The study focused on reactions only to Forest Labs’ Celexa, and didn’t find any link between the genetic variations and suicide attempts, The New York Times reports. This distinction is critical, because docs don’t know whether people who report suicidal thoughts are at increased risk to act on them. The one patient in the study who attempted suicide denied suicidal thoughts, the paper writes.

The findings are likely to enlarge an ongoing controversy over the safety of antidepressants. Two years ago, the FDA required drugmakers to place Black Box warnings on product labels warning of suicidal behavior and thoughts. The American Psychiatric Association, however, complains the warnings and publicity have scared away some docs and patients, robbing some people of a useful treatment.

“What I would say is that this study is a wake-up call, that we may have the opportunity to use genomic tests to guide personalized care for depression,” said Tom Insel, director of the National Institute of Mental Health, which helped finance the study, tells the paper. But he adds that the genetic test “is not yet ready for prime time.” Indeed, perhaps one antidepressant will one day be proven safer for some people, but not others.

The researchers used data from a large government-financed depression study that included more than 4,000 adult patients, and found that about 6 percent reported having suicidal thoughts of suicide after taking Celexa, usually within the first few weeks of starting treatment. They then analyzed blood samples from 120 who reported suicidal thoughts to see whether variations in certain genes were common.

The scientists found that 36 percent of the patients who had markers for two gene variations reported suicidal thoughts - which was more than 10-fold risk compared with those with neither of the gene markers. These patients were also far less likely to recover taking the drug. Both markers were in genes that affect how the brain processes a chemical messenger called glutamate, which works to activate neurons.

If genetic tests are developed, “they could add to the whole clinical picture,” Francis McMahon, chief of the genetics and mood disorders unit of the National Institute of Mental Health and the senior author of the study, tells the Times. “If a patient tells me he thinks his life is not worth living, or I know he’s at risk of having that reaction, I’m going to monitor him more closely or treat him differently.”

The researchers searched for links to 68 genes in the patients’ blood samples that might influence mood states, but these are clearly not the only genes that could be involved in the emergence of suicidal thoughts. Many of the patients who reported a sudden urge to end their lives did not have either of the gene variations found to put people at high risk.