Michael Wood says in his documentary The Story of India first episode:
Soma is still used as a medicine in Central Asia. The active element in the plant is ephedrine, and the effect that it has, according to the Rig Veda is, well, if you take too much of it, it can cause nausea, it can be frightening, it can give you vertigo, sickness, vomiting. If you take it in the right measure, it enlivens the senses, sharpens you up, keeps you awake. The poets in the Rig Veda compose their songs often at night having drunk Soma, and, of course, Indra, King of the Gods, drinks vast quantities of this perhaps because it's thought to be an aphrodisiac as well. [...] But Soma's not an Indian plant.It doesn't grow in the humid plains. And today, it's no longer part of Hindu religion. It came from outside.
And then afterward in the documentary someone suggested that it can contain also cannabis. Someone on the PBS site asks a similar question:
Is there any historical proof either primary or secondary other than the above mentioned that the Vedic Soma might have contained cannabis or even ephedrine?