Our results show that dual HER2 blockade treatment is associated with a significantly increased risk of severe diarrhea (OR 2.52, p<0.001) and treatment discontinuation (OR 1.52, p=0.014), but not for severe rash (OR 1.06, p=0.81), liver toxicities (OR 1.16, p=0.28), CHF (OR 1.46, p=0.09), LVEF decline (OR 1.09, p=0.40) and FAEs (OR 0.97, p=0.91). Here, ERBB2 is linked to congestive heart failure.