What was the doctor monkey joke from friends?

The full joke about the doctor/monkey is never revealed on the Friends episode entitled The One With The Joke (season 6, episode 12). The only clues we have about the joke that Ross had published in Playboy are that the doctor is a monkey, he seemed to have written some sort of prescription, and Monica thinks it is offensive and a horrible joke.

Both John Cryer and Jon Favreau were offered the role of Chandler Bing. Janeane Garofalo and Jami Gertz were offered the role of Monica, while Courtney Cox was originally offered the role of Rachel.

It is reported that Jennifer Aniston (Rachel) enjoys watching Friends (1994-2004) reruns (it makes her happy) and has ruled out the idea of a reunion movie.