Subcutaneous (SC) injection applications are the process of administering the drug to the loose connective tissue under the skin and is one of the most frequently applied parenteral drug administration methods among nursing practices. SC injections, which are frequently performed by nurses, often result in complications such as hematoma, ecchymosis and pain at the injection site. Although the field of healthcare is developing day by day, injection applications can still be painful and uncomfortable. These procedures increase the individual's stress levels before the procedure, causing them to experience fear and anxiety and reducing patient satisfaction. This may cause the application to fail or be repeated. Many nonpharmacological methods are applied to prevent or reduce these local side effects in subcutaneous injection applications. Cold application is accepted as an effective and easy-to-use method for SC injection. Cold application may also constrict peripheral blood vessels, reducing blood flow to the tissue and thus preventing ecchymosis. When the national and international literature is examined, there are studies using cold application to reduce pain and ecchymosis due to SC injection in adults. This study aimed to evaluate the effectiveness of cold injection, which is considered an alternative to cold therapy, in reducing pain and ecchymosis due to SC injection application. No study has been found in the literature evaluating the effectiveness of cold injection in preventing pain and ecchymosis due to SC injection application. Therefore, it is thought that this study will make a significant contribution to the literature and create a simple, comfortable and cost-free innovation. Preparation and use of cold injection is simple, easy and comfortable. It is anticipated that patients will be able to easily follow the procedure steps during the procedure. Therefore, this study aimed to examine the effect of the cooled needle on the development of pain and ecchymosis in SC injection application in adults.