Obtaining high-quality analgesia in prehospital patients with severe pain is an important treatment objective for medical team. Opioids are recognized as the treatment of choice for relief of severe acute pain. Recommended initial analgesia of patients with severe acute pain, defined as a numerical rating scale (NRS) score of 6/10 or higher, in a prehospital setting in France consists of the administration of morphine by the medical staff of mobile intensive care units. The intravenous administration of morphine is usually considered as the gold standard for any emergency setting. Nevertheless, even when using high dose boluses and aggressive titration protocols, pain relief remains inconsistent for some patients. Moreover, unexpected acute pain in an emergency setting causes anxiety, insecurity, and stress that can diminish perceptions of control over pain and diminish ability to decrease pain. This may interfere with the opioid treatment and poses the question of whether an anxiolytic drug added to the morphine titration could relief pain and/or anxiety.There is no study, to our knowledge, assessing the clinical efficacy of hydroxyzine added to the morphine in a prehospital setting. Hydroxyzine is used primarily as an antihistamine drug for the treatment of itching, allergies, motion sickness-induced nausea, and insomnia, as well as notably for the treatment of mild anxiety. This randomized double-blind group clinical trial is designed to determine the efficacy and safety of hydroxyzine in addition to a conventional intravenous morphine titration protocol by comparing hydroxyzine and placebo for medical prehospital treatment of adult patients with severe traumatic acute pain. Eligible patients with a numerical rating scale (NRS) score of 6/10 or higher will be randomly allocated to receive either hydroxyzine 1mg/kg at inclusion (group A) or an intravenous placebo. The intravenous morphine analgesia will be given and titrated according to the pain score every 5 minutes. The drugs will be administered by the physician from syringes of similar appearance prepared by the nurse who is not otherwise involved in the study. The protocol-defined primary outcome measure is the percentage of patients with pain relief (with a NRS score of 3/10 or lower) 15 minutes after the first injection. Secondary outcomes include pain score comparisons every 5 minutes within the first 30 minutes and comparison of adverse events. The physician blinded to the analgesic treatment groups will do all assessments of patients. The safety evaluation will include non invasive monitoring of blood pressure, heart rate, respiratory rate, oxygen saturation by pulse oximetry (Spo2), and a sedation scale (0, patient is awake; 1, patient is with intermittent sleeping; 2, patient is sleeping, awakened by verbal stimulation; 3, patient is sleeping, awakened by tactile stimulation; 4, patient is not aroused by stimulation) at these periods. Fifteen minutes after the first injection, overall patient and investigator satisfaction with analgesia will be recorded. Moreover, several assessments will be done by psychologists 2 and 21 days after the trauma, including post traumatic stress and anxiety levels.