Copyright © 2015 Wolters Kluwer Health, Inc.There are few studies evaluating the effect of ShotBlocker on pain acquired from intramuscular injection, and these are mostly in children. We hypothesized that the use of ShotBlocker, while administering an intramuscular injection, would reduce the pain and anxiety due to intramuscular injection in adults. A randomized, placebo controlled trial was carried out for more than 20 months in 2010-2011 on 180 adults aged 18 to 80 years who received intramuscular injections of diclofenac sodium (75 mg/3 mL) at the outpatient clinic of a hospital. The patients were grouped into 3 groups: control, placebo control, and experimental. The experimental group was given an intramuscular injection of diclofenac sodium with ShotBlocker. Pain intensity was measured through a visual analog scale after the injection and anxiety was measured using Spielberger's State-Trait Anxiety Inventory. Pulse rate was counted and state and trait anxiety was measured before and after the injection. The Mann-Whitney U test and Wilcoxon and Kruskall-Wallis tests were used to evaluate the data. Patients in the ShotBlocker group had significantly lower pain intensity than those in the placebo and control groups. State anxiety level increased after the injection in the experimental group but did not change in the other 2 groups. ShotBlocker did not affect the pulse rate. Our results suggest that using ShotBlocker during intramuscular injection reduced patients' pain intensity because of injection but did not reduce anxiety levels. Thus, ShotBlocker is recommended as a pain-relieving tool during intramuscular injection in adults.