Just an OD on insulin. There are also ways to strike the chest that will disrupt the heart’s rhythm and cause it to tear itself apart (Learned about from sensei/brother-in-law, now deceased from heart attack [I didn’t do it]).
Succhonylcholine is used by anesthesiologists to paralyze the lungs briefly prior to intubating the patient. If used on paraplegics, it will immediately cause a heart attack, and an excessive amount might cause respiratory failure, but I never heard of it causing heart attacks in non-paraplegics. Then again, I’m no doctor, and that might work, too.