Depends on the poison. Although you would probably be right in this case since Magellan's poison was probably quite inorganic and its (bio)chemical functionality was most likely of quite fundamental nature (considering it was melting even rocks). So, no amount of developed immunity might help against it.

However, you only would be right because in the One Piece universe flesh is infinitely stronger than any lifeless substance, toxic materials included. If you are tough enough, eating Polonium for dinner is no problem.