After letting my docs learn about the plants I put them on other tasks, research, farming etc. Sometimes they arent' happy about it but with some persuasion (repeatedly dropping them on the spot) they will eventually get the message.

Once I did have one villager who wouldn't work ANYWHERE until I let her check out all the herbs. Once she became a trainee doc then she was willing to work wherever she was needed. Weird...Guess she really wanted to become a doctor.