In Horizon, Cally gives Vila a shot, he reacts with great pleasure, and she has to TELL him that it's adrenaline and soma--so presumably this is his first contact. Yet very shortly afterwards, when Cally reiterates to Blake that they need a rest, Blake says "What about the old adrenaline and soma?" and Cally says that they've "practically been living on it."

Vilakins may be self-administering by mouth; he wouldn't much like needles, would he? And the feel of a injected drug might be different from the same thing swallowed as a liquid-- different absortion rates, you know.