Perhaps the carbonation?
When i do propylhexedrine extractions I use carbonated water and citric acid powder. Carbonated water seems to help a bit. Not sure if it actually does, but it seems to.
However I always take another step and remove the nasty lavender oil that is in the liquid. Lavender oil is actually quite pharmacologically active, especially in the large amounts present in those inhalers, and imparts a dirty spacey feel to the propylhexedrine experience.
But it can be easily be removed. Make your extraction as usual (pepsi works fine too), pour it into a jar and add half a cup of cooking oil (canola oil, olive oil, any oil). Then put a lid on it and shake the mixture vigorously for a minute or two. Allow it to settle for a minute or two, and then use a large syringe to carefully siphon off the oil layer that has settled on top (dump this oil). The remaining liquid should be relatively free of lavender oil (since it is now "bound" to the cooking oil). I actually use a separatory funnel to separate the two liquids but I've used a large syringe before.
It feels much cleaner this way.
But watch out for propylhexedrine, it isn't something that should be taken too often due to the heavy cardiovascular strain it imparts. It is also likely a somewhat effective serotonin releasing agent in high doses, which is not something you want to be doing very frequently