Difference between a 'medicine' and a 'medicinal'

What is the exact difference between the words "medicine" and "medicinal"?

For medicinal, Free Dictionary states:

"A preparation or product having the properties of a medicine."

But the fact is that a "medicine" is itself a "preparation" or "product" which has curative properties. Is there any particular reason for making a noun out of this adjective?

Aside from the noun/adjective difference already mentioned, I would say the main difference (in modern usage) between a medicine and a medicinal is that a medicine's primary, inherent use is as a curative. A medicinal, on the other hand, is a substance that is being pressed into service as a curative - it may not normally be viewed as a medicine.

For instance:

• an antibiotic is a medicine
• a shot of brandy could be a medicinal, if given to treat shock, etc. (not that that's necessarily a good idea!)

That's the main difference in my mind, but feel free to disagree!