Both scenarios have lots of potential for great storytelling. When choosing between them, consider what kind of story you'd most like to tell, and which of the two is going in a direction you find more interesting/compelling.
Let me throw some light on the primary differences between the two avenues you're suggesting.
If the drug is a secret, then your tale is one of conspiracy. The story will necessarily revolve around the pursuit of truth - finding out the truth about the drug, fighting the government which tries to cover it up, trying to bring awareness to the population at large. In your specific case, you have a very nice twist, in that the population - including the protagonist - is drugged to reject the conspiracy theory, and not to fight against it. That's an additional obstacle to overcome.
There's an implicit assumption here that people would oppose the conspiracy if they only knew about it (otherwise, it wouldn't need to be secret). A conspiracy story provides a clear division between good guys and bad guys - the bad guys are the ones secretly manipulating everybody; the good guys are the ones trying to fight back.
Another consideration to take into account is that conspiracy provides natural outlets for exposition. Explaining a central SF-nal concept is always a challenge; a conspiracy tale lets readers understand the concept right along with the protagonists, as they uncover it. You've still got your work cut out for you - you've got an insidious drug to develop, a massive conspiracy, and a whole society thrown out of whack! - but since your story's primary focus is finding out more about what's going on, you've got a lot of good opportunities to explain important information without breaking pace.
Your other option is that your SF-nal element has already been accepted as the status quo. In this case, your story becomes one of revolution. It's a story that revolves more around argument, persuasion, and a deeper understanding of why and how the society believes what it does, and why this society is fundamentally mistaken. It's likely to be a story focused on the society, not only on plot and action. This is because what you're describing is a huge change to society - everybody knows that society has changed drastically; just describing everyday actions will need to make considerations of how different this new society is. It's a change that's too big to ignore, so it'll probably get a fair share of the spotlight.
With conspiracy, I mentioned a natural good/evil conflict. Here you're going to have to work harder for that, because either you've got a nation of morons supporting something blatantly stupid (which is seldom fun or interesting...) or else the opposing opinion is semi-legitimate, and gets at least some measure of sympathetic portrayal. In other words, the conflict here can be more subtle, with more shades of gray. (In your particular example, you add a nice twist because the question of agency arises - to what extent can you argue with people drugged to be content? This can come into play in both versions, but the social-oriented one might take the question farther and deeper, addressing it in more complex ways, and not as a mere obstacle.)
And when it comes to exposition, well, you're trying to portray an unusual but oblivious society - that might be pretty tough, but can be very rewarding! Here you've got less excuse to focus on exposition than you do with the conspiracy version - you don't have the easy inroads. But a story focused on social elements will have advantages of its own - you might decide to focus on more character interactions, on relations within the society, since that's more appropriate to the social focus.
So, in conclusion, I think you'll find a conspiracy is a more straightforward, conflict-based story; a revolution is subtler and more complex. Of course, these are all generalities - e.g. you could have a conspiracy story that dealt heavily with the society created by the conspiracy, or you could take the story in a completely different direction than what I've outlined.
But these, I think, are (some of) the immediate considerations that arise naturally from this particular choice. Hope this helps!