It didn’t occur to me that it might be a placebo, but that scene did strike me as odd. Honestly, would you just drink a potion made by Mr. Gold without some sort of second thought? Seriously? Redeemed or not, I still don’t trust him. David had no choice, he had to trust, but still.
David did seem a tiny bit frisky after drinking it though. Maybe Gold spiked that potion with a little something he (Gold) might need at a later date.