Inclusion Criteria:  Written informed consent. Both genders, ages 18-55 years (older patients may not tolerate high ACZ dose). PANSS total score > 60 and Score > 4 on one or more items of the 'positive' syndrome items (P1-P7), following treatment at therapeutic doses for 6 weeks with different APDs on 2 occasions. Stable dose of antipsychotic drug (APD) for > 1 month, continued throughout the study. Not participating in another randomized controlled clinical trial (RCT).  Exclusion Criteria:  Substance abuse in the past month/dependence past 6 months, (except nicotine). History or current medical/neurological illnesses that may lead to unstable course, e.g., epilepsy. Pregnancy. Acetazolamide (ACZ) contraindications: hypersensitivity to ACZ; history of renal hyperchloremic acidosis; Addison's disease/adrenal failure; chronic closed angle-closure glaucoma. Current or prior treatment with ACZ or history of hypersensitivity to ACZ. Intellectual disability as defined in DSM 5.