You are having air bubbles inside the syringe (medicine cartridge), right?

I've encountered one Forteo pen which had one small bubble. Did not really know where it came from or when it first appeared.. there did not appear a way to get rid of it. I used that pen without any further problem - the little bubble stayed in the pen throughout the remaining injection period.

One thing I can assure you: each pen has more than 28 doses, actually about 3-4 extra doses after you complete 28 day injection. However, the manufacturer says to discard the pen after 28 days of use. I think you could keep an eye on those bubbles in the pen. If more bubbles appear or existing ones get bigger which may be indicative of a faulty pen, contact the specialty pharmacy or manufacturer to see if you could get a replacement.