“ Now they have hemp growing in their land, which is very like flax except in thickness and in height, for in these respects the hemp is much superior. This grows both of itself and with cultivation; and of it the Thracians even make garments, which are very like those made of flaxen thread, so that he who was not specially conversant with it would not be able to decide whether the garments were of flax or of hemp; and he who had not before seen stuff woven of hemp would suppose that the garment was made of flax.

75The Scythians then take the seed of this hemp and creep under the felt coverings, and then they throw the seed upon the stones which have been heated red-hot: and it burns like incense and produces a vapour so thick that no vapour-bath in Hellas would surpass it: and the Scythians being delighted with the vapour-bath howl like wolves.[72] This is to them instead of washing, for in fact they do not wash their bodies at all in water.”

Herodotus.

— Now Herodotus is not exactly a reliable historian, but I don’t think there is any reason for him to have just invented this.

Also, “medicinal” use wasn’t really a category that they would have recognized, nor was there a taboo on intoxication that would make them invent such a category. There might have been ritual element to it, or some other instrumental reason behind it, but I think a contemporaneous description of them seeming to enjoy it is pretty good reason to believe that part of it was that it was just a good time.