Heavy metal concentrations of Pb, Cd, Cr, and Cu leaching from single-use plastic cups were identified, and the risks associated with them were assessed in real time (up to 10 min). All samples (tea, carbonated beverage, and lassi) were placed in disposable plastic cups and held for less than 1 min, 5 min, and 10 min, respectively. Prior to digestion, the solids were ashed in a muffle furnace at 550 °C for 30 min. The filtrates were then tested for heavy metals. The samples were all confirmed to be contaminated with heavy metals. Heavy metals leached from the plastic cups in the following order: Cu > Pb > Cr > Cd. The samples' HI values were less than one, hence there was no evidence of a non-carcinogenic risk. The ILCR values for this heavy metal contamination in samples exceed 10−3, indicating a high carcinogenic risk. Lassi poses the highest possible carcinogenic risk. A rise in temperature and a drop in pH also resulted in heavy metal migration. Heavy metal leaching from plastic cups poses a serious health risk due to its toxicity. The proposed legislation should prohibit or restrict the serving of warm and hot beverages in plastic cups.