Background: Intramuscular injections (IMIs) remain a frequent practice in mental health. Few studies have examined the issue of nurses' practices concerning IMI in this domain, and none considered specifically hygiene. Finally, no study appears to have looked at emergency situations and their possible influence on practices. The principal objective of our study was to assess the practices associated with IMI in mental health, especially the hygiene-related practices. The secondary objectives were as follows: 1) to assess the practices for the preparation and administration of IMI in mental health for criteria other than hygiene and 2) to determine whether professional experience and emergency situations influence these practices. Design: Single-centre cross-sectional study in a psychiatric Hospital and adherence to the STROBE guidelines. Methods: The participating nurses all worked with inpatients, and all volunteered for the study. A self-administered questionnaire was sent to all participants. The questionnaire included questions about knowledge and practices associated with IMI, some considered in two different situations: emergencies and planned injections. The distribution of the responses was tested by the chi-squared test or Fisher's exact test, as appropriate, or by McNemar's chi-squared test or Friedman's nonparametric chi-squared test for matched data. Results: Response rate was 48.6%. Overall, 81% of nurses reported correct handwashing before preparation, 87.5% responded that the dorsogluteal site is currently recommended for IMI, and 74.6% that they did not know the "Z track" technique. In planned injections, 58.7% reported that the choice of needle was determined, at least in part, by the patient's body mass index. In emergency situations, adherence to guidelines was less frequent for all types of practices. Conclusions: This study shows the need to improve practices for the frequently used procedure of IMI among mental health nurses. Relevance to clinical practice: Better professional education appears necessary to develop optimal practices, especially in emergency situations.