Sarafem belongs to a class of medications called selective serotonin reuptake inhibitors. To put it simply, this means that Sarafem selectively blocks serotonin from being taken up by certain nerve cells in the brain.

Doctors are still trying to understand what causes PMDD and depression, but they have learned that an imbalance of serotonin in the brain is a factor. Serotonin is a natural brain chemical called a neurotransmitter that helps nerve cells communicate with each other.

Sarafem helps to correct a serotonin imbalance by increasing the brain's own supply of serotonin. As the balance returns, symptoms ease off.

Older medications, called tricyclic antidepressants, also restored serotonin balance. At the same time, however, they interfered with two other neurotransmitters -- norepinephrine and dopamine. That often caused some unpleasant side effects.

Because Sarafem works only on serotonin, it has far fewer and less severe side effects.