The word serotonins is playable in Words With Friends™, no blanks required. Because it is longer than 7 letters, you would have to play off an existing word or do it in several moves.
Words With Friends™ Letter Score: 12
Highest Scoring Words With Friends™ Play In The Letters
Seven Letter Word Alert: (32 words)
erosion, inserts, interns, intoner, intones, introns, ionones, nonriot, noosers, norites, nosiest, notions, orients, osteons, rosiest, seniors, sinners, sinters, sonnets, sooners, soonest, sootier, sorties, stoners, stonier, stories, tension, tensors, tinners, toonies, torsion, triones