BACKGROUND: Heparin is an anticoagulant medication that is normally injected subcutaneously. Subcutaneous administration of heparin may result in complications such as bruising, haematoma and pain at the injection site. One of the factors that may affect pain, haematoma and bruising is injection speed. OBJECTIVES: To assess the effects of the duration (speed) of subcutaneous heparin injection on pain, haematoma and bruising at the injection site in people admitted to hospitals or clinics who require treatment with unfractionated heparin or low molecular weight heparin. SEARCH METHODS: The Cochrane Peripheral Vascular Diseases Group Trials Search Co-ordinator searched the Specialised Register (last searched August 2013) and CENTRAL (2013, Issue 7). We searched MEDLINE, EMBASE, CINAHL and two Persian databases Iranmedex and SID (August 2013). SELECTION CRITERIA: We sought randomised controlled trials (RCTs) comparing the effects of different durations of subcutaneous injections of heparin on pain, bruising and haematoma at the injection site. DATA COLLECTION AND ANALYSIS: Two review authors, working independently, extracted data onto a structured form and assessed study quality. We used the criteria recommended by the Cochrane Handbook to assess the quality of included studies. The study outcomes were summarised using quantitative and qualitative methods. MAIN RESULTS: One RCT was identified which met the inclusion criteria, involving 50 participants with a mean age of 55.25 (± 12.37) years. In this trial it was not possible to blind the participants and care givers. The method of sequence generation and allocation concealment was not described. The overall quality of the evidence was moderate due to the single small included study. Each participant had two injections, one in the left side and one in right side of the abdomen. One of these was injected slowly (intervention) and the other was injected fast (control). The second injection was 12 hours after the first injection. The duration of fast injection was 10 seconds and the duration of slow injection was 30 seconds. The study reported a significantly lower pain intensity for slow versus fast injection. The mean pain intensity was 13.9 ± 17.1 mm with the slow injection and 20.6 ± 22.3 mm with the fast injection (P < 0.001). In addition the bruising sizes were smaller with slow injections compared to fast injections at 48 hours follow-up (mean bruising size 18.76 ± 9.32 mm(2) with the slow injection and 109.2 ± 468.66 mm(2) with the fast injection, P = 0.033) and 72 hours follow-up (mean bruising size 21.72 ± 76.16 mm(2) with the slow injection and 110.12 ± 472.86 mm(2) with the fast injection, P = 0.025). The incidence of haematoma was not measured as an outcome. AUTHORS' CONCLUSIONS: There is only limited evidence of any difference in pain intensity and bruising sizes following slow versus fast injections due to the inclusion of only one small unblinded trial. The single included study suggests that slow injection might have slightly lower pain intensity and bruising size at the heparin injection site, but the results should be considered with caution. Until more reliable evidence emerges, slow injection might be the preferred approach.