Irrigation with wastewater imperils human health by introducing toxic metals, harmful pathogens, and organic and inorganic pollutants into crops that potentially contaminate the entire food chain. This pot experiment investigated the impact of Faisalabad wastewater (Madhuana drain, Pharang drain, university of agriculture) and canal water (control) on lettuce (Lactuca sativa L.), focusing on morphophysiological parameters, which were assessed alongside biochemical stress markers such as photosynthetic pigments, enzymatic antioxidants, oxidative and secondary metabolites. Water physiochemical analysis showed that the Madhuana drain contained the maximum heavy metal concentration, including lead (1.5 mg/L), cadmium (0.13 mg/L), chromium (0.83 mg/L), and zinc (5.46 mg/L), then other irrigation treatments but all wastewater sources beyond the recommended permissible limit. The growth results indicated that Madhuana drain wastewater irrigation significantly altered and decreased plant growth patterns (plant fresh weight (19%) and plant dry biomass (22%), leaves per plant (26%), leaf area (40%), and plant height (45%), and also increased metal accumulation, including Pb (3.75 mg/kg), Zn (12.92 mg/kg), Cd (1.21 mg/kg), and Cr (2.05 mg/kg), while photosynthetic pigments revealed a significant reduction, with chlorophyll content declining by 40%, 31%, and 60% than all other irrigation treatments. Oxidative stress markers including MDA, EL, and H2O2 were elevated, indicating cellular damage by 64%, 62%, and 73%. Enzymatic antioxidant assays (peroxidase, superoxide dismutase, and catalase) and secondary metabolites (flavonoids, alkaloids, phenolics, and anthocyanin) elevated by 50% and 40%, respectively, indicating a response against heavy metal stress. These findings underscore that untreated wastewater adversely influences plant metabolic mechanisms that ultimately reduce growth and physiological processes. To protect food security and yield, strict wastewater treatment procedures must be put in place to reduce environmental contamination.