Is it unsafe / poisonous to use advil in this way?

Well, according to the label you should 'Use only as directed'. So it's not recommended for you to use Advil in that way. I'm not sure I'd view it as harmful, unless you are exceeding the recommended dosage. However, I'm not a pharmacist or doctor, so don't go by what I say. I would highly recommend calling up a pharmacist and speaking to them about this. I do know that if you are doing this for an extended period of time (as in, say, a few weeks), you could end up causing yourself some harm.