Lower back pain on one side: causes, red flags and when to get it checked

The lower back contains the lumbar spine, discs, facet joints, muscles and nerve roots, and also sits next to the kidneys, which lie higher at the back under the lower ribs. The pain location, character and what you do to relieve it help tell apart the spine from other organs.

• Non-specific mechanical back pain: muscle or ligament strain, often after lifting, twisting or prolonged sitting, better with gentle movement.

• Facet joint or sacroiliac joint irritation: one-sided pain worse on extension or when standing from sitting.

• Disc prolapse with nerve irritation: back pain with leg pain, tingling or weakness (sciatica).

• Muscle spasm and poor posture or inactivity.

• Kidney stones: severe colicky pain in the flank spreading to the groin, with blood in urine and nausea.

• Urinary or kidney infection: loin pain, fever, burning on urination.

• Inflammatory back conditions such as axial spondyloarthritis in younger adults with morning stiffness that eases with movement.

• Spinal stenosis, vertebral fracture from osteoporosis, infection or tumour, and in women gynaecological causes such as endometriosis or ovarian cysts.

Numbness in the saddle area, new difficulty passing urine or controlling bowels, progressive weakness in a leg, fever, unexplained weight loss, a history of cancer, severe night pain unrelieved by position, pain after significant trauma, age under 20 or over 55 with new pain, and long-term steroid use are red flags.

• Call 999 or go to A&E: new bladder or bowel loss, numbness between the legs, or rapidly worsening leg weakness (possible cauda equina syndrome).

• Same-day GP or NHS 111: back pain with fever, loin pain with urinary symptoms, or severe colicky pain.

• GP within days: back pain with leg weakness, or pain at night that does not ease.

• Routine GP or physiotherapy: back pain beyond about six weeks, or recurrent episodes.

The doctor asks when it began, what you were doing, the pain quality, whether it spreads, whether it is relieved by movement or rest, night pain, leg symptoms, urinary and bowel function, fever, weight loss and medical history. They examine posture, spine movement, tenderness, leg strength, reflexes and sensation, straight-leg raising, and kidney area tenderness.

• Usually none for mechanical pain; imaging does not change management in most cases.

• Urine dipstick: for infection or blood in the urine.

• Blood tests: inflammatory markers or kidney function where indicated.

• MRI lumbar spine: for suspected nerve compression, persistent pain with red flags or before procedures.

• CT KUB or ultrasound: for suspected kidney stones.

Stay active and keep moving as comfort allows; bed rest is not helpful. Use heat, simple painkillers if suitable, and gradually return to normal activities. Physiotherapy exercises, stretching and strengthening help recovery and reduce recurrence. Avoid prolonged sitting and lift with bent knees.

The NHS route is a GP and physiotherapy, with spinal or pain clinics for persistent problems. Private physiotherapy, osteopathy and spinal consultations are available directly, with MRI where indicated.