Oleander is a poisonous plant! These people are putting it in their bodies!
No, these people are putting an aqueous extract of oleander which has had the toxins removed in their bodies. A great many things in life are toxic unless properly prepared and consumed. Water itself will kill you dead as can be if you drink enough of it. So, I suspect, would be silver if you had a huge bowl full of silver powder and kept eating it by the cupful.
And if not, you would have fun trying to get through the metal detectors at the airport.