Schizophrenia (SZ) extracts a heavy personal and public health cost, primarily because there is no effective treatment. Though many drugs are currently available, the majority provide only partial relief for psychotic phenomena and none guarantee more than modest relief for 'negative symptoms' or for cognitive impairments. The Investigators must search for additional effective and safe medications. Recently, big data analytic strategies have yielded numerous 'repurposed' drugs, i.e., drugs with new indications that are already licensed for other uses. These strategies utilize massive data bases of known drug effects to find candidates that could predictably counteract known pathogenic effects of the disorder in question. Repurposed drugs are appealing not only because they have already been marketed and have known side effect profiles, but also because they have increased prior probability of efficacy. Still, careful randomized controlled trials (RCTs) are necessary for the new indications. The investigators have designed a systematic search for repurposed drugs likely to be beneficial for patients with SZ. Our novel search strategy began with the construction of a comprehensive protein-protein interaction network (PPI) for SZ using a validated method. Next, The Investigators searched public data bases for drugs that have predicted effects on multiple proteins in the SZ PPI network, but opposite to those observed in patients with SZ. The initial list was pruned using predetermined criteria, leaving 7 drugs of which cromoglycate (CGY) had the best negative correlation score. Reassuringly, three other drugs with lower scores in our list have already been tested for SZ. CGY is a safe and highly effective mast cell inhibitor that has been licensed for over 25 years for prophylaxis of asthma and allergies; it is also used to treat systemic mastocytosis and ulcerative colitis. Independent of our research, CGY is also predicted to stabilize the blood brain barrier (BBB), which can be disrupted in patients with SZ. Animal studies and favorable Log P estimates assure that CGY can cross the BBB. CGY has few reported side effects, despite its extensive use. Thus, multiple factors motivate our RCT. The Investigators propose a double blind adjunctive RCT for SZ using CGY. To maximize therapeutic benefits while minimizing risk and discomfort, The Investigators will enroll outpatients with SZ who meet criteria for residual positive symptoms after adequate trials of standard antipsychotic drug (APD) therapy (N=100, total). The Investigators will prefer patients in the early course of their illness. CGY or placebo will be added to prescribed medications for 4 weeks utilizing the Sequential Parallel Comparison Design to maximize power. The primary outcome will be improvement in positive symptoms as determined by the Positive and Negative Syndrome Scale (PANSS) positive symptom subscale. Secondary outcomes include total symptoms (PANSS total score), negative symptoms (PANSS negative symptom scale scores), cognition (Penn Computerized Neurocognitive Battery), and social function. Serum CGY levels will be monitored. The Investigators have proven experience with RCTs and the large number of patients are our clinical service ensures that recruitment targets will be fulfilled.