Left-side back pain is overwhelmingly caused by muscle strain or joint irritation, the same culprits behind most low back pain in general. But the left side of your back also sits near your left kidney, your spleen, your pancreas, part of your colon, and your aorta, so pain in that location occasionally signals something that has nothing to do with muscles or bones. The challenge is telling the difference between a sore muscle that will resolve on its own and an organ problem that needs prompt attention.
Muscle and Joint Problems Are the Most Common Cause
The vast majority of left-side back pain comes from the same musculoskeletal sources that cause back pain anywhere: strained muscles, irritated joints, or tight connective tissue. One muscle that deserves special mention is the quadratus lumborum, a deep muscle on each side of your lower spine that connects the bottom rib to the pelvis. When this muscle develops painful knots called trigger points, the result is an aching, sometimes sharp pain on one side of the back that can be hard to pin down. In patients with nonspecific low back pain, trigger points in the quadratus lumborum are a well-recognized finding on clinical exam.1PubMed. The occurrence and inter-rater reliability of myofascial trigger points in the quadratus lumborum and gluteus medius: a prospective study in non-specific low back pain patients and controls in general practice These trigger points respond to hands-on treatment approaches, including muscle energy techniques and strain-counterstrain methods, both of which have shown meaningful improvements in pain and function.2Pakistan Journal of Medical and Health Sciences. Effectiveness of Muscle Energy Technique Versus Strain Counter Strain Technique on Trigger Points of Quadratus Lumborum among Low Back Pain Patients
Beyond trigger points, the facet joints (the small paired joints that connect each vertebra to the next) can become inflamed or arthritic on one side, producing pain that stays localized to the left or right of the spine. Disc bulges or herniations can also compress a nerve root on one side, sending pain down the buttock and leg. These musculoskeletal causes share a few reassuring features: the pain usually changes with position (bending, twisting, or sitting makes it better or worse), you can often recall a triggering activity, and there are no alarming symptoms like fever, weight loss, or blood in your urine.
Kidney Stones and Kidney Infections
Your left kidney sits behind the lower ribs in the back, right about where many people point when they say their “back hurts.” Kidney stones lodged in the left ureter produce a distinctive pain pattern: sudden, severe, wave-like pain that starts in the flank and radiates toward the groin. One clinical sign doctors check for is costovertebral angle tenderness, which is pain when the area where the lowest rib meets the spine is tapped firmly. In a study of patients suspected of having ureteral stones, this exam finding helped distinguish those who actually had stones from those who did not.3PubMed Central. Evaluation of the usefulness of costovertebral angle tenderness in patients with suspected ureteral stone
Kidney infections (pyelonephritis) also cause left-sided back pain, typically alongside fever, chills, and painful urination. In patients with acute focal bacterial nephritis, costovertebral angle tenderness is present in a meaningful proportion of cases, though its absence does not rule out infection.4PubMed. Skin-to-Renal Pelvis Distance Predicts Costovertebral Angle Tenderness in Adult Patients with Acute Focal Bacterial Nephritis The combination of back pain, fever, and urinary symptoms should prompt an urgent medical visit. Untreated kidney infections can spread to the bloodstream.
Gastrointestinal Sources of Left-Side Back Pain
Several digestive organs sit on or near the left side of the body and can refer pain to the back when inflamed. The pancreas stretches across the upper abdomen, and pancreatitis (especially chronic pancreatitis) produces a deep, boring pain in the upper abdomen that frequently radiates straight through to the back. This pain is often worsened by eating or drinking and can be constant or come and go.5PubMed Central. Pathogenesis and management of pain in chronic pancreatitis The back component of pancreatic pain is often left-sided or central, and it can easily be mistaken for a spinal problem if the abdominal symptoms are mild.
Diverticulitis, an inflammation or perforation of small pouches in the colon wall, tends to affect the left (sigmoid) colon. It usually presents with left lower abdominal pain, but it can also produce left-sided back pain. In one reported case, a patient with perforated diverticulitis initially presented with generalized malaise and left lower back pain as prominent symptoms, along with loss of appetite.6Gazeta Médica. Perforated Diverticulitis Complicated by Pylephlebitis and Splenic Infarction The takeaway is that if left back pain comes with changes in bowel habits, fever, or abdominal tenderness, the colon could be the source rather than the spine.
Vascular Emergencies That Mimic Back Pain
This is the category that makes “when to worry” such an important question. An abdominal aortic aneurysm (AAA) is a dangerous bulging of the body’s largest artery, which runs along the spine. When it expands or begins to leak, it can cause low back pain that is remarkably easy to confuse with an ordinary back strain. In one case report, a patient presented with what seemed like routine low back pain: he could not identify a specific injury, his symptoms did not change with position, and nothing made the pain better or worse. The key finding was a strong pulsation felt through the abdomen; imaging revealed an aneurysm measuring up to 5.5 cm, large enough to require intervention.7PubMed. Abdominal aortic aneurysm in a patient with low back pain
When symptomatic, aortic aneurysms tend to present with abdominal pain, flank pain, and sometimes blood-flow problems in the legs.8PubMed Central. Abdominal Aortic Aneurysm: An Overlooked Etiology of Low Back Pain The critical red flags here are back pain that doesn’t change with movement, that came on without an obvious physical trigger, that is accompanied by a sense of pulsing in the abdomen, or that occurs in someone over 60 with a history of smoking or high blood pressure. A ruptured aortic aneurysm is a surgical emergency with a very high mortality rate, so these warning signs warrant immediate medical evaluation.
Pulmonary Embolism and Pleural Problems
The lungs and their lining (the pleura) extend into the upper back. A blood clot that travels to the lungs, known as a pulmonary embolism, can cause sharp pain in the back or shoulder, especially on the affected side. In one case, a patient developed left shoulder and left-sided back pain roughly two weeks after abdominal surgery. The pain had a pleuritic quality, meaning it worsened with breathing. She was ultimately diagnosed with a left-sided pulmonary embolism.9PubMed Central. Pulmonary Embolism Presenting As Shoulder and Back Pain: A Case Report
Pneumonia, pleurisy (inflammation of the lung lining), and a collapsed lung can all cause one-sided back pain, typically in the upper or mid-back. The giveaway is that the pain tends to be linked to breathing: it sharpens when you inhale deeply or cough. Shortness of breath, a new cough, or a racing heart alongside back pain should be taken seriously, especially if you have recently been immobilized (long flight, surgery, bed rest) or have a history of blood clots.
Gynecological and Reproductive Causes
In women, the reproductive organs sit in the pelvis and can refer pain to the lower back. Ovarian cysts, endometriosis, uterine fibroids, and ectopic pregnancies can all produce one-sided back pain. Ovarian cysts occasionally grow large enough to compress nearby structures, producing pain that can seem unrelated to the pelvis. In one documented case, a young woman presented with left-sided back pain that turned out to be caused by a giant ovarian cyst on the opposite (right) side; the cyst had grown large enough to cause compression and a deep vein thrombosis in her left leg.10PubMed Central. Diagnosis and resection of a giant ovarian cyst presenting in a young patient with contralateral back pain and lower limb deep vein thrombosis While this is an extreme example, it illustrates how pelvic conditions can produce misleading pain patterns.
Pregnancy itself is a common cause of left- or right-sided back pain, particularly in the second and third trimesters. Pelvic girdle pain affects a large proportion of pregnant women and was long attributed to the hormone relaxin loosening the pelvic joints. A systematic review of this theory, however, found low-quality evidence for a link between pelvic girdle pain and relaxin levels.11PubMed Central. Pregnancy-related pelvic girdle pain and its relationship with relaxin levels during pregnancy: a systematic review The causes are likely a mix of postural changes, weight redistribution, and altered movement patterns rather than a single hormonal trigger.
Shingles and Other Nerve-Related Causes
Herpes zoster (shingles) can produce severe, burning left-side back pain before any rash appears, sometimes leading to misdiagnosis as a muscle strain or kidney problem. The varicella zoster virus, the same one that causes chickenpox, lies dormant in nerve clusters along the spine and can reactivate later in life. The resulting nerve pain follows a band-like pattern along one side of the body, and it predominantly affects older adults and people with weakened immune systems.12Neuropathic Pain. Herpes Zoster and Postherpetic Neuralgia
The tricky period is the first few days, before the characteristic blistering rash erupts. During this pre-rash phase, the pain can be intense and one-sided with no visible explanation. If you develop a burning or stinging pain on one side of your back that wraps around toward your chest or abdomen, and especially if you are over 50 or have a condition that suppresses your immune system, shingles should be on the radar. Once the rash appears, the diagnosis becomes straightforward, and early antiviral treatment can reduce both the acute pain and the risk of long-lasting nerve pain afterward.
Spinal Infection and Cancer Warning Signs
Spinal infections are uncommon but serious, and they can present as persistent, worsening back pain. In a review of back pain patients ultimately diagnosed with spinal infection, the most common clinical features were spinal pain (present in about three-quarters of cases) and fever (present in over half).13PubMed Central. Red flags for the early detection of spinal infection in back pain patients Diabetes and intravenous drug use were the most common risk factors. A separate study in Nigeria found that a history of tuberculosis, unexplained weight loss, and radicular pain (shooting pain into a limb) were strongly associated with spinal infection.14PubMed. Red flags for spinal pain in patients diagnosed with spinal infection in Nigeria: A 10-year medical records review
Cancer that has spread to the spine (metastatic disease) shares many of these red flags. Pain from spinal tumors tends to be constant, progressive, and present at night, often waking you from sleep. It does not improve with rest and may be accompanied by unintended weight loss, a history of cancer, or new neurological symptoms such as weakness or numbness in the legs. Any combination of unrelenting back pain, systemic symptoms like fever or weight loss, and a relevant medical history (cancer, immunosuppression, recent infection, IV drug use) should prompt imaging and blood work rather than a wait-and-see approach.
Osteoporosis and Compression Fractures
In older adults, particularly postmenopausal women, a compression fracture in the thoracolumbar spine can cause sudden one-sided or central back pain without any major trauma. These fractures happen when weakened vertebrae partially collapse under normal body weight or after something as minor as a cough or bending to pick up an object. Low back pain from osteoporosis-induced compression fractures is a common and disabling problem.15PubMed Central. Spinal Dorsal Rami Injection and Radiofrequency Neurolysis for Low Back Pain Caused by Osteoporosis-Induced Thoracolumbar Vertebral Compression Fractures
A Polish study comparing postmenopausal women with and without vertebral fractures found that those with fractures reported substantially more severe back pain (average pain scores of about 6 out of 10 versus 4 out of 10), and their pain occurred more frequently, lasted longer, and interfered more with daily activities.16PubMed Central. Comparison of the Characteristics of Back Pain in Women with Postmenopausal Osteoporosis with and without Vertebral Compression Fracture If you are a postmenopausal woman (or anyone with known osteoporosis) and you develop new, sharp back pain that started suddenly and is centered around one area of the spine, a compression fracture should be considered even if there was no fall or obvious injury.
When Back Pain Becomes Chronic
Most episodes of left-side back pain improve within a few weeks, but sometimes pain lingers well beyond the expected healing time. When low back pain persists past about three months without an identifiable structural cause, the nervous system itself may be part of the problem. Research into central sensitization has shed light on how the brain and spinal cord can become overly reactive to pain signals, amplifying sensations that would normally be mild or unnoticeable. This process can turn normal or subthreshold stimuli into persistent pain in people who may have certain psychological predispositions, such as chronic stress, anxiety, or depression.17PubMed Central. Is the Central Sensitization in Chronic Nonspecific Low Back Pain Structural Phenomenon or Psychological Reaction? A Narrative Review
This does not mean the pain is imaginary. It means the nervous system has learned to stay in a heightened alert state, and the pain is real even if there is no ongoing tissue damage. Recognizing this pattern matters because the treatment shifts away from looking for a structural fix (more imaging, more injections, more surgery) and toward strategies that calm the nervous system: graded exercise, cognitive behavioral therapy, sleep improvement, and stress management. If your left-side back pain has lingered for months, does not match any of the organ-related patterns described above, and your imaging is unremarkable, central sensitization may be worth discussing with your doctor.
How Doctors Figure Out Which Category You Are In
When you show up with left-side back pain, a clinician’s first job is to sort you into one of three buckets: likely musculoskeletal (no red flags, mechanical pain pattern), possibly organ-related (features suggesting kidney, GI, vascular, or pelvic involvement), or potentially serious spinal pathology (signs of infection, fracture, or cancer). Your history does most of the heavy lifting. Pain that changes with movement and started after physical activity is almost certainly musculoskeletal. Pain accompanied by fever, urinary symptoms, or abdominal complaints points toward internal organs.
When imaging is needed, the choice between ultrasound and CT depends on the suspected diagnosis. A comparison of the two modalities found that CT was more sensitive than ultrasound for detecting both acute appendicitis and acute diverticulitis.18PubMed Central. A comparison of the Accuracy of Ultrasound and Computed Tomography in common diagnoses causing acute abdominal pain For kidney stones, CT is typically the first-line imaging choice because of its high accuracy. For suspected musculoskeletal problems, imaging is usually unnecessary in the first four to six weeks unless red flags are present. Early imaging for routine back pain has not been shown to improve outcomes and can lead to incidental findings that create unnecessary anxiety.
A Practical Red-Flag Checklist
You do not need to memorize the full differential diagnosis. Instead, watch for specific features that move left-side back pain out of the “probably muscular” category and into territory that deserves same-day or emergency medical evaluation:
- Fever or chills: Suggests infection, whether in the kidney, spine, or elsewhere.
- Blood in urine: Points to kidney stones, infection, or, less commonly, a kidney or bladder tumor.
- Unintended weight loss: A red flag for cancer or chronic infection like tuberculosis.
- Night pain that wakes you: Pain that is worst at rest and disrupts sleep is more concerning for tumor or infection than for muscle strain.
- Pain unrelated to movement: Musculoskeletal pain almost always changes with position. Pain that stays the same no matter what you do suggests an internal source.
- New leg weakness or numbness: Indicates nerve compression that may require urgent intervention, especially if accompanied by difficulty controlling your bladder or bowel.
- Abdominal pulsation: A strong, palpable pulse in the abdomen alongside back pain raises concern for aortic aneurysm.
- Sudden severe pain with shortness of breath: Raises the possibility of pulmonary embolism or aortic dissection, both of which are emergencies.
- Recent surgery or immobilization: Increases the risk of blood clots that can cause back and chest pain.
If none of these features are present and your pain behaves mechanically (it came on after physical activity, it changes with position, and it is improving day to day), you are almost certainly dealing with a muscle or joint issue that will resolve with activity modification, gentle movement, and time. When in doubt, a call to your doctor or a visit to an urgent care clinic can sort things out quickly with a focused exam and, if needed, basic lab work or imaging.