This is a key consideration in the design of AAI and in prescribing decisions. There is a concern that owing to increasing rates of obesity and gender differences in thigh tissue depth, needle lengths in currently available AAI devices may not be adequate to ensure effective delivery of adrenaline into the thigh muscle.

In order for adrenaline to be effective it is important for it to be administered intramuscularly; subcutaneous administration can result in a slower rate of absorption and lower peak adrenaline plasma concentrations.

Skin to muscle depth - Gender differences and inter-patient variation in skin to muscle depth (STMD), along with an increasing prevalence of obesity, may mean that currently available AAI cannot guarantee intramuscular administration.

Females tend to have a thicker subcutaneous layer on the thigh than men and obesity results in fat deposition around the thigh increasing the STMD.

Injecting technique - there are two key methods of self-injecting adrenaline with an AAI; either a €swing and jab€ method or a €place and press€ method. The method of administration is partly determined by the activation force required for the individual device. It can also result in a tissue compression effect reducing the skin to muscle distance.