A small change

The patient shook his doctor's hand in gratitude and remarked, "Since we're the best of friends, doc, I wouldn't want to insult you by offering payment. But, you'll be pleased to know that I've mentioned you in my will.""That's very kind of you," replied the doctor. After some quick thought, the doctor added, "May I see that prescription I just gave you?""Sure, but why?" asked the patient.The doctor replied, "I'd like to make a small change..."