I do the same, yes its wasteful but when you look at the sharp boxes I fill up regularly, making my life easier by removing the need to do two injections is more important . It would make little difference to the waste I already produce. Waste production is just what we diabetics do

. I don’t think there is much left after you have given your last unit, could be wrong I have never tried to extract it. They should try to make diabetes more environmentally friendly, but to be honest I would rather they spent that money on a cure, solving the waste problem and all.
