BaSO₄ is relatively harmless, but BaS is highly toxic.
BaSO₄ is quite insoluble (240 µg/100 mL). It is a mild irritant in cases of skin contact and inhalation. However, it is safe enough that health professionals ask patients to drink a suspension of BaSO₄. The Ba is opaque to X-rays, so it makes the stomach and intestines more visible to radiographers.
BaS is soluble (7.7 g/100 mL). It reacts slowly with water and more rapidly in the acid conditions of the stomach to release H₂S.
BaS + 2HCl ⟶ BaCl₂ + H₂S
An H₂S concentration of 60 mg/100 mL can be fatal within 30 min.
Don’t eat barium sulfide!