There is evidence suggesting that pen needles must be primed for full effectiveness and to ensure that insulin is delivered correctly.
A study published in 2010 the Journal of Diabetic Sciences and Technology states the following:
Pens must be primed before each injection, and the needle removed immediately after each use.11 This is performed by instructing the patient to dial up 2 units and inject these units into the air (also called an “air shot”). This will displace any air in the needle and ensure an accurate injection. This air shot may need to be repeated when using a new pen or cartridge until a steady stream of insulin is observed. Insulin pens are manufactured with enough extra insulin to account for this air shot. An insulin pen must never be used by more than one individual, even if the pen needle is changed, because sharing of insulin pens can result in the transmission of hepatitis viruses, human immunodeficiency virus, or other blood-borne pathogens.49