This was a different kind of puzzle, and I was happy to see the response. I got correct answers from Mahi Saraf, Sahaj Sharma (my nephew from Jaipur) and Pratik Poddar – well done all. Thanks again to Alok Mittal for contributing this puzzle.
Here is the answer:
Sage’s plan: If the sage drinks a very weak poison before coming to the king and carries a random drink saying it is his poison (which actually has no poison) he will not die as first he will first have a weak poison and then the strong one from the pharmacist to survive. Next as his own drink has no poison, and the pharmacist drinks his own strong poison, the pharmacist will die.