@Bubba, the pens go down to 1 unit; not sure why you would want half a unit. Just double the back water when you reconstitute and use 1 unit. I think Jeff covered this on YouTube https://www.youtube.com/watch?v=geNpx5sJH3k. Basically, if you use as little as 1 unit, the little drip at the end of the needle contains more peptide than if you were using 5 units. I normally reconsitiute things so i'm doing 10units....Unless it's GHK-CU or something that stings, then maybe 20 units.