The title 劇薬中毒 means something like "addicted to a powerful drug", and =LOVE commit to that metaphor for the whole song. The person she loves is the medicine: 「きっと あなたは苦くて甘い薬」 (surely you are a bitter, sweet medicine), and worse, 「あなたしか効かない」 (only you work on me). It's a love song written like a pharmacy leaflet, and I think that's the fun of it.
The details are what sell it. She kills time at a convenience store near his place, and notices the only lipstick running low is the one he once complimented, which she reads as her answer. Then the drug talk comes back with warnings: 「強い副作用に気をつけて」 (watch out for strong side effects) and 「一日一度が限度のお薬で」 (a medicine with a limit of once a day). She even admits 「警報はずっと前から鳴っている」 (the alarm has been ringing for a long time). She knows it's a bad romance and keeps going anyway, which is a bit much, and also very honest.
Learner tip: 劇薬 is a real word for a powerful or dangerous drug, and 副作用 (side effect) is the same word you'd see on actual medicine boxes in Japan.
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