why do doctors write Rx before prescribing medicine?

The Rexell sign came much later, LOL, Rx did not come from the sign.

Here is one web search:

"Rx" means "prescription for medicine." The letters abbreviate the Latin word recipe, which is a form of the verb "to take."

Doctors write Rx in the heading of prescriptions as an instruction to "take" the medicine. The pharmacists filling the orders understand this shorthand (and hopefully they can read the doctors' handwriting) and print it on pill bottles with whatever else doctors order, such as "take twice daily with food." Somewhere along the line, pharmacists started using "Rx" on their storefront signs so patients knew where to get their doctors' instructions translated.

Another possible meaning for "Rx" suggests that it's a form of the astrological symbol for the Roman god Jupiter, written on prescriptions to get the god's blessing. However, Jupiter's symbol bears only a faint resemblance to "Rx." Although Jupiter was something of a protector figure, mythology doesn't connect him with medicine or healing.

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Actually Rx is not short for Prescription........it's short for "Remedy"......thus the Big R with the cross in the bottom of the Rs leg.....it's a symbol....Long ago that symbol was the trademark for Rexall Drug stores. The orignal symbol was supposed to be an R with just a long mark like "/" across the bottom of the right leg of the R to form a "Y" so it would form an R and Y but it ended up with an R and X instead.....How do I know this?......I'm a retired artist who started out as a sign painter, I had to learn that stuff from a book...Yeah they had books on sign painting back when I was a kid.....a long time ago.

rx is a latin word which means "to take" that is why it is written before medicines referring that they should be taken.some also mean it as prescription.

Rx is short for prescription.....

i Feel its to remember one great Doctor, may not be indian Dr.