Inclusion Criteria:  French speaking Suffering from [chronic low back pain OR fibromyalgia] for >6 months Average pain at least 4/10 Central sensitization (assessed via a standardized physiotherapy evaluation and the Central Sensitization Inventory)  Exclusion Criteria:  Health condition for which propranolol is contra-indicated Medication with which co-administration of propranolol is contra-indicated Severe or uncontrolled neurological/psychiatric condition (including post-traumatic stress disorder, substance abuse, suicidal ideation, etc.) Surgery of the lower-back in the last 3 years Litigation surrounding the painful condition