Acute low back pain is a common cause for emergency department visits. Controversy remains regarding the optimal medication for acute low back pain relief. The investigators hypothesized that administration of pharmacological anxiolysis in addition to analgesia will improve pain relief and patient management in the emergency department.

Acute low back pain is a common problem in the emergency department and pain relief is usually the first step in patients' management. Numerous medication options are available for acute LBP relief,each class of medication has its associated benefits and harms.Controversy remains regarding the optimal analgesic treatment.Anxiety has been found to be a predictive factor of pain intensity in patients with acute low back pain and anxiolysis by non-pharmacological measures has been shown to have a positive effect on pain management in the ED setting.

Promethazine is a first-generation H1 receptor antagonist of the phenothiazine chemical class used commonly as an antihistamine antiemetic. It has a strong anxiolytic-sedative effect and its safety and efficacy in managing anxiety related to medical procedures is well documented.It may be reasonable to assume that pharmacological anxiolysis with promethazine may assist in alleviation of acute pain in the strenuous environment of the ED.

Inclusion Criteria:

1. An indication for opioid analgesia based in the ED (i.e. severe pain > 70mm on a 100mm VAS)
2. Age between 18-65 years
3. American Society of Anesthesiologists (ASA) score of 1 or 2, and no preexisting glaucoma, cardiac arrhythmia or pulmonary disease
4. Systolic blood pressure higher than 90 mmHg on admission
5. Willingness and ability to provide an informed consent
6. No known hypersensitivity to the medication used.

Exclusion Criteria:

1. Pregnant women
2. Patients who can not be under adult supervision following discharge from the emergency department.