You might be surprised to learn that SSRIs act in part by altering the metabolism of hormones in PMDD, and they actually work MORE consistently for treating PMDD than for depression. (For PMDD, SSRIs work about 60% of the time, which is actually really good in the medication world!). There are several randomized controlled trials of SSRIs for treating PMDD showing that SSRI beats placebo even after just a few days of treatment.
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". 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 seem to be the best first-line tool we have for treating PMDD.
Updated 22 December, 2018 by Tory Eisenlohr-Moul, PhD & Liisa Hantsoo, PhD