TenForward: Where Every Vulcan Knows Your Name

I'd say Symbiosis was the DARE episode:

Wesley Crusher: Data, I can understand how this might happen to the Ornarans. What I can't understand is this: Why would anyone voluntarily become dependent on a chemical?

Lt. Commander Data: Voluntary addiction to drugs is a recurrent theme in many cultures. Regrettably, having had no firsthand experience with such dependency, chances are I cannot give you an appropriate response.

Wesley Crusher: That's all right, Data.

Lieutenant Tasha Yar: [jumping in] I believe I can, Wesley. In the first place, nobody wants to become dependent. That happens later.

Wesley Crusher: Still, if people know it happens, why do they even start?

Lieutenant Tasha Yar: Remember what I told you about life on my home planet? There was so much poverty and violence that, for some, the closest thing to an escape was through drugs.

Wesley Crusher: How does a chemical substance provide an escape?

Lieutenant Tasha Yar: In the real sense, it can't... but it makes you think it's doing so. You have to understand that drugs - can make you feel good. They put you on top of the world. You're happy, sure of yourself, in control...

Wesley Crusher: What's the point, though, if you know it's artificial?

Lieutenant Tasha Yar: Because it doesn't seem artificial until after the drug wears off. Then comes what they call a "crash". You feel just the opposite of the way you did earlier: sad, insecure, like everything's coming down on you and there's nothing that can be done about it.

Wesley Crusher: [DELETED LINE] Doesn't that wear off, too?

Lieutenant Tasha Yar: [DELETED LINE] Ultimately, yes - but the trouble is that a lot of people can't wait for "ultimately". It's got nothing to do with common sense; they simply don't have enough willpower to ride out the crash. So they take more of the drug to cope, to feel the way they did before. The problem with doing that is, your body adapts to the drug as you keep taking it.

Wesley Crusher: [DELETED LINE; doing the math] ... Meaning you'd need larger and more frequent dosages to get the results you wanted, wouldn't you?

Lieutenant Tasha Yar: Indeed. On the other hand, using more of the drug more often also leaves you feeling worse - and for longer - after it wears off. That's how you get trapped. Before you know it, you're taking the drug... not to feel good, but to keep from feeling bad. At that point, all you care about is getting your next dosage. Nothing else matters.

Wesley Crusher: I see how it all works, Tasha, but I'm still not sure I understand it. Sorry.

Lieutenant Tasha Yar: Wesley. Don't be. In fact, let's hope for your sake you never do.