A joke in the traditional manner

My dad, who turns eighty-seven today, is still turning out jokes. To wit: What did the doctor tell his forgetful patient to do?

No spoilers. The punchline is in the comments.

More jokes in the traditional manner
The Autobahn : Did you hear about the cow coloratura? : Elementary school : A Golden Retriever : How did Bela Lugosi know what to expect? : How did Samuel Clemens do all his long-distance traveling? : How is cod shipped to a supermarket? : What did the plumber do when embarrassed? : What happens when a senior citizen visits a podiatrist? : Which member of the orchestra was best at handling money? : Why did the doctor spend his time helping injured squirrels? : Why did Oliver Hardy attempt a solo career in movies? : Why did the ophthalmologist and his wife split up? : Why was Santa Claus wandering the East Side of Manhattan?

[“In the traditional manner”: by or à la my dad. He must take credit for all but the cow coloratura, the squirrel-doctor, and Santa Claus.]