The number of units depends on two things : how strong the vial is (its concentration in mg/mL) and how much you were prescribed (your dose in mg)
Argyropoulou D, Geladas ND, Nomikos T, Paschalis V
Shaking creates strong shear forces in the liquid that can physically break the peptide chains apart or cause them to unfold and aggregate (clump together), destroying their biological activity
Potential concerns include appetite stimulation, which may counteract weight-loss goals, and ghrelin-mediated effects on cortisol and sleep
Venkova K, Mann W, Nelson R, Greenwood-Van Meerveld B
10.1016/j.jaci.2012.03.013 120