The antidepressant Effexor should be taken in the smallest possible dose to reduce the risk of overdose, U.S. regulators said Wednesday.

The drug's manufacturer, Wyeth, and the U.S. Food and Drug Administration have received reports of overdoses in people taking Effexor, also called venlafaxine.

The overdoses were most common when the drug was taken with alcohol or other drugs, the agency said.

Doctors should prescribe the "smallest quantity of capsules consistent with good patient management," the FDA said in releasing a letter from Wyeth.

The letter did not say how many overdoses have been reported. The most commonly reported events in overdoses were a faster-than-normal heart rate, changes in level of consciousness, widening of the pupils, seizures and vomiting.

It's unclear whether the increased risk of overdose can be attributed to the drug or characteristics of the patients taking it.

Antidepressant overdoses can be potentially fatal. The risks with Effexor may be higher than other selective serotonin reuptake inhibitors such as Prozac, Zoloft and Paxil, but less than with older antidepressants called tricyclics, the company said.

Effexor maximizes levels of two messengers in the brain, serotonin and norepinephrine, while SSRIs work on serotonin alone.

Earlier this year, Health Canada warned women taking SSRIs who are pregnant or thinking of becoming pregnant that the drugs may carry potential risks to the baby.