Well, I'm no Niklas and won't even pretend to know how he thinks, but my guess would be: both. In general, it seems he really likes to write lyrics with multiple meanings and multiple ways to interpret. Of course, Lethe could be an exception and perhaps he had a definitive meaning in his mind when he wrote it, but I doubt it.
Furthermore, I believe both the interpretations you mentioned could be combined: Love (and some other extreme emotions) can often be almost narcotic, and certainly addictive. When in love we drift into another world, so to speak, a world we sometimes can't leave behind, not even when we are no longer in love. And when we are not in love, we tend to (ab)use alcohol to get away from this reality, to forget this loveless world just for a short moment.
-Villain