Needles Subcutaneous fat a b s t r a c tBackground: The objectives of the study were to estimate the following in adults of Indian origin: a) Gender and side differences in the skin-to-muscle (SM) and muscle-to-bone thickness (MB) at the deltoid intramuscular injection site; b) Correlation of SM thickness with the BMI, age and gender; c) The prevalence of under and over-penetration assuming a standard needle length of 25 mm and following prescribed guidelines for IM injection. Methods: The SM, MB and skin-to-bone (SB) thicknesses were bilaterally estimated in two hundred adult Indian subjects (100 male and 100 female) using an ultrasound probe at a pre-determined point on the upper arms of the subjects. The BMI of each subject was calculated. The unpaired sample 't' test and paired 't' test were used to analyse differences between groups. Pearson's correlation coefficient was used in correlation analysis and suitable linear regression equations were generated. Results: Females had a significantly higher SM thickness and lower MB thickness. The SM thickness was significantly greater on the left side, while the SB and MB thickness were significantly greater on the right. Multiple linear regression equations for both the dominant and non-dominant arms had good model fit properties. Under-penetration would have occurred in 2 (1%) subjects while over-penetration would have occurred in 50% of the subjects. Conclusion: Over-penetration of deltoid IM injections is likely to be more prevalent as compared to under-penetration. Therefore, the technique of IM injection needs to be modified based on the body type of the individual patient.© 2014, Armed Forces Medical Services (AFMS). All rights reserved. m e d i c a l j o u r n a l a r m e d f o r c e s i n d i a 7 0 ( 2 0 1 4 ) 3 3 8 e3 4 3http://dx