Her answer didn't seem to mention the Society of Apothecaries, though. But where did you get the report that they used the sign of Jupiter?
I possibly didn't look hard enough on their website, but I couldn't find anything about the Jupiter sign (or the Rx symbol).
The site does explain their coat of arms, granted as stated by James I in 1617 (the original Grant of Arms is shown in a photograph here
) in the following way :
It features Apollo (the god of healing) killing the dragon of disease, supported by two unicorns (from King James's royal arms), and a rhinoceros as the crest (the powdered horn was believed to be medicinal). The motto, from the first book of Ovid's 'Metamorphoses', translates: "I am spoken of all over the world as one who brings help"
(For those wishing, a better view of the coat of arms is on their homepage (http://www.apothecaries.org/