Enhancement of neurocognition is essential in the treatment of schizophrenia. In our previously reported six-week Randomized Controlled Trial (RCT) mirtazapine added to conventional antipsychotics improved not only negative, but also positive symptoms and neurocognition in difficult-to-treat schizophrenia. The present study aimed to explore whether a prolonged exposure to mirtazapine could further improve neurocognition.Completers of the RCT who were able and willing to proceed to the extension phase received open label mirtazapine for an additional 6weeks. During the extension phase, both groups (i.e., patients who previously received mirtazapine and those who received placebo) and the whole population showed improvement on a number of neurocognitive tests. Patients who shifted to open label mirtazapine from placebo achieved in the six following weeks similar results as their initially mirtazapine-treated counterparts did during their first 6weeks of mirtazapine exposure. Middle-term mirtazapine treatment (12weeks) demonstrated an advantage over short-term mirtazapine treatment (6weeks) on Stroop Dots time and Trail Making Test, part B, number of mistakes (t=−2.562, p=0.035 and t=−2.42, p=0.043, correspondingly).Mirtazapine added to antipsychotics consistently shows desirable effects on neurocognition. Lengthy treatment seems worthwhile. Mirtazapine may become a safe and cost-saving neurocognitive enhancer in schizophrenia, yet more studies are needed.