An analysis of records from the National Poison Data System from a 10-year period ending in 2011 identified 294 reported incidents of people developing adverse symptoms after ingesting 35% (high-concentration) hydrogen peroxide (H202). Among these, 41 (13.9%) experienced an embolic event such as seizure, stroke, or pulmonary embolism, and 20 (6.8%) died or became disabled as a result. Other symptoms included altered mental status, respiratory distress, and hypoxia (lack of blood in the tissues). Consumption of H202 can be dangerous even in small amounts -- one of the deaths reported occurred from drinking just 15 ml (0.5 oz) of hydrogen peroxide liquid (Hatten, 2017 Ann Emerg Med).
For more information, see our CL Answer about drinking hydrogen peroxide.