https://medbasic.co.uk/ What size of Insulin Do You Really Need? Needles are determined by their length, gauge, or the thickness. Doctors advise needles to be between 4 and 6 millimeters in length. For almost all body types, there's no need to use needles that are longer than 8 millimeters. Needles that are longer can penetrate too deep, delivering an intramuscular injection. Insulin injections into muscle can boost insulin absorption and could cause hypoglycemia. A lot of people believed that the needle's size ought to be adjusted according to the body's weight. This meant that people who were overweight required larger needles. A larger needle could be beneficial to some with overweight or other medical issues such as arthritis, but recent research has proven that overall body weight does not affect needle size effectiveness according to an article that was published in Diabetes Spectrum.