Interactions between bufadienolides derived from toad venom and verapamil in langendorff-perfused guinea-pig hearts

► Verapamil worsens heart block and lethal bradycardia of bufadienolides in vitro. ► Verapamil increases the uptake of bufadienolides in heart tissue. ► P-gp inhibitors and the drugs containing bufadienolides may produce dangerous interactions.