SSRIs are an evidence-based treatment for PMDD
SSRIs work for the majority (about 60%) of people with PMDD. Like with any treatment, individual differences in treatment response are very important to acknowledge. So when we say "SSRIs work for PMDD," what we really mean is that "SSRIs work better than a placebo (sugarpill) for about 60% of people with PMDD". This is about the same percentage as for major depression. We definitely don't intend to imply that SSRIs work for ALL people with PMDD, but we also don't want to downplay the importance of SSRIs for PMDD, since they are the best first-line tool we have for treating PMDD. 

Continuous Dosing vs. Luteal-Phase Dosing
When using SSRIs for PMDD, there are different dosing schedules. These include continuous dosing (medication is taken daily) and luteal phase dosing (medication taken daily during luteal phase). Usually, the SSRI doses used to treat PMDD are lower than doses used to treat major depression. Also, SSRIs usually relieve symptoms faster in PMDD (within a day or two) than for major depression (can take 3-6 weeks for SSRI to work). 

How do SSRIs work for PMDD?
We are still learning about how exactly SSRIs help to reduce symptoms in PMDD. One possible mechanism is that SSRIs alter the metabolism of hormones in PMDD. Another possibility is that individuals with PMDD are more sensitive to the minuscule increases in serotonin (a brain chemical that helps modulate mood) that occur in the days following SSRI initiation.

-Liisa Hantsoo, PhD, June 28, 2019