WASHINGTON — Epilepsy drugs used by millions of people may increase the risk of suicidal thoughts or behavior, the Food and Drug Administration warned in an alert to doctors Thursday.

The FDA analyzed almost 200 studies of 11 different anti-seizure drugs, some that have been on the market for decades. The studies tracked almost 28,000 people given the medications and another 16,000 given dummy pills.

Very rarely were suicidal thoughts or behavior reported. Still, the FDA found drug-treated patients did face about twice the risk: 0.43 percent of drug-treated patients experienced suicidal thoughts or behavior compared with 0.22 percent of placebo-takers.

Overall, four people in the drug-treated groups committed suicide, and none in the placebo groups.

What that means: For every 1,000 patients, about two more drug-treated patients experienced suicidal thoughts than placebo-takers, FDA concluded.

Anti-seizure drugs are used for a variety of illnesses in addition to epilepsy, including migraines, certain nerve-pain disorders, and psychiatric diseases such as bipolar disorder that themselves carry a risk of suicide.

The FDA found drug-treated patients were at increased risk no matter their diagnosis, but that the risk was highest for epilepsy sufferers.

The FDA began investigating if epilepsy drugs pose any suicide risk in 2005. It analyzed data from 11 well-known anti-seizure drugs including Pfizer Inc.'s Neurontin, Novartis AG's Tegretol and Abbott Laboratories' Depakote — but the FDA said it expected that the risk applied to every epilepsy drug. The FDA said it would work with manufacturers to add the warning to product labels.

The FDA did not respond to requests for comment about how patients should interpret the warning.

But the agency's letter to doctors advised them to:

— Balance the risk with the patients' need for the drug.

— Tell patients and their families about the risk so they can be aware of changes in mood.

— Make sure patients and families know to contact a doctor if someone experiences common suicide warning signs, such as talking or thinking about hurting yourself, becoming preoccupied with death, withdrawal, becoming depressed or worsening depression, and giving away prized possessions.