IntroductionVenous thromboembolism (VTE) is the leading cause of preventable hospital deaths. Adults hospitalised with psychiatric illness vary in their risk of VTE, and therefore in their likelihood of benefiting from thromboprophylaxis. There is a paucity of evidence-based practice guidelines addressing VTE prophylaxis for this population despite recognition of additional VTE risk factors in this population.AimTo develop an evidence-based guideline on VTE prophylaxis for patients hospitalised with psychiatric illness using Grading of Recommendations, Assessment, Development and Evaluation (GRADE).MethodAn international, multidisciplinary, guideline panel including clinical experts, methodologists, and a patient partner was recruited by invitation. Panelists were selected based on methodological and clinical expertise on this subject. Panel members were diverse in geography (from Ireland, the United Kingdom, France, and Canada), expertise and gender. The panel was composed of four advanced specialist psychiatric pharmacists, four consultant haematologists, four consultant psychiatrists, one advanced nurse practitioner in psychiatry, one advanced nurse practitioner in anticoagulation, a methodologist with expertise using GRADE, and a patient partner with lived experience of VTE. The panel prioritised two clinical questions and related population, interventions, outcomes, and secondary analyses according to their importance for patients. GRADE was used to assess certainty of evidence and to move from evidence to risk-stratified recommendations.ResultsThe panel made three recommendations: a strong recommendation against parenteral pharmacological prophylaxis for patients at low risk of VTE (moderate-certainty evidence); a conditional recommendation in favour of parenteral pharmacological prophylaxis in high-risk patients (low-certainty evidence); and a strong recommendation against graduated compression stockings in patients at high risk of VTE with a contraindication to parenteral pharmacological prophylaxis (low-certainty evidence).ConclusionClinicians should not use parenteral pharmacological prophylaxis in adults hospitalised with psychiatric illness at low risk of VTE; and should consider using parenteral pharmacological prophylaxis for high-risk adults with no contraindications. Graduated compression stockings are not recommended in high-risk patients when parenteral pharmacological prophylaxis is contraindicated. These GRADE- based recommendations offer one of the first evidence-based practice guidelines for thromboprophylaxis decisions in psychiatric in-patient settings.