Inclusion Criteria:  Patients meeting the DSM-V criteria for schizophrenia spectrum disorders. Clinical Global Impression (CGI) score ≥ 4 and ≤ 6 at screening. Initiated treatment with a stable dosage of typical and/or atypical antipsychotic medication for at least four weeks.  Exclusion Criteria:  Previous sensitivity to pentoxifylline (PTF). Chronic immune and/or inflammatory diseases (such as rheumatoid arthritis, systemic lupus erythematosus, chronic inflammatory bowel disease). Consumption for > 3 consecutive days of any immune-modulating or anti-inflammatory drug in the last month. Current active and persistent substance and/or alcohol abuse. Any severe, unstable medical condition (e.g., cardiovascular disorders, diabetes mellitus, respiratory diseases, cancer). Obesity (body mass index > 30). Cognitive dysfunction such as retardation. Known or suspected pregnancy or breastfeeding women. Lactose intolerance or sensitivity.