High amounts (several grams daily over several days) of lapachol can cause uncontrolled bleeding, nausea, and vomiting.⁹ Use of the whole bark is typically safer than isolated lapachol—side effects have included nausea and gastrointestinal upset.¹⁰Pregnant or breast-feeding women should avoid use of pau d’arco.

One case report exists of a 28-year-old man who died of liver failure after taking unspecified amounts of pau d’arco, scullcap, and zinc.¹¹ It appears likely that this may have been a case of adulteration of scullcap with germander.¹²