Why Does My Back Hurt on One Side Only?
Back pain on one side only usually means the muscles, joints or ligaments on that side are under more stress than on the other side. This often happens because the pelvis is tilted or rotated, one leg is slightly longer, or a single joint such as the SI joint is irritated. A strain in the left lower back or an ache on the right is rarely about that spot alone. It is usually part of a wider alignment pattern.
Key points
- Back pain on one side is commonly caused by a difference between the two sides of your pelvis, hips or legs, not just a local muscle problem.
- Scans done lying down can miss these alignment problems because gravity and body weight are taken away.
- A standing, weight-bearing image of your whole body can show where the imbalance actually starts.
- Most back pain on one side is not dangerous, but some symptoms need urgent medical attention.
What does back pain on one side actually mean?
It means the discomfort is clearly on the left or right of your spine, not across the middle. You might describe it as a strain in your left lower back that tightens after sitting, or a deep ache on the right that flares up when you stand up. The pain can feel muscular, sharp, or like a constant dull pull.
Your spine does not work on its own. It sits on top of your pelvis, and your pelvis sits on top of your legs. If anything in that chain is uneven, the muscles and joints on one side of your back end up doing more work. Over time, that side starts to complain.
How can you tell if your one-sided back pain is linked to alignment?
Pain linked to alignment tends to follow patterns. It is usually worse on the same side every time, changes with posture or activity, and does not fully go away with rest alone. Here are the common signs.
The ache is always on the left or always on the right. It does not switch sides.
When you are upright, gravity puts more load on the uneven side, so symptoms increase.
The muscles on that side feel constantly tense, even after stretching.
You notice one hip sits higher, or your belt line is not level.
What causes pain on one side of the lower back?
Several things can put more load on one side of your back than the other. The most common causes relate to how your skeleton lines up when you are standing and moving.
When one side of the pelvis sits higher, the spine curves to make up for it. This is a common cause, and it is often hidden. Learn more about lateral pelvic tilt.
Even a small difference in leg length shifts your weight to one side. Read about leg length discrepancy.
The sacroiliac joint connects the pelvis to the spine. When it is inflamed on one side, the pain centres there. See our page on SI joint dysfunction.
A twist in the pelvis loads the muscles on one side unevenly. Find out how a rotated pelvis affects your back.
Other possible causes include a mild scoliosis (a sideways curve in the spine), muscle guarding, where one side tightens to protect an area, or strain in the quadratus lumborum, a deep muscle that runs from the pelvis to the ribs on each side.
Is back pain on one side related to the kidneys?
Sometimes. Kidney pain tends to sit higher, under the ribs towards the back. It may come with other symptoms, such as a change in how you pass urine, a fever, or feeling generally unwell. If you have any of these along with your back pain, see your doctor. If the pain is lower and changes with movement or posture, it is more likely to be musculoskeletal.
When should you see a doctor urgently for back pain on one side?
Most back pain on one side is not dangerous. But some signs need prompt medical attention. Contact your doctor, call your local urgent medical advice line, or go to the emergency department if you have any of these along with your back pain:
- Numbness or tingling in your legs, groin or buttocks
- Weakness in one or both legs
- Loss of bladder or bowel control
- Sudden, severe pain that woke you from sleep and does not ease
- Unexplained weight loss or a fever
These could point to something that needs urgent tests. Do not wait for a scan. Get medical help straight away.
How does a standing scan show what lying-down scans miss?
Standard X-rays, CT and MRI are usually done lying down. When you lie flat, gravity is taken away, your muscles relax, and your skeleton settles into a neutral shape. A tilt, rotation or leg length difference that causes your pain when you are upright and carrying your body weight may not show up at all.
Standing EOS imaging captures your entire skeleton from head to feet in a natural, weight-bearing position. It shows exactly how your pelvis, spine and legs line up in real life, the way you actually stand and move. If one side is taking more load, it shows clearly.
Lying-down scans are excellent at showing individual structures, but they take away the very forces that create your symptoms. A pelvic tilt or a small leg length difference may flatten out on the table. The report then comes back looking normal, even though you are still in pain.
At ScanAlign, full-body standing imaging uses EOS technology, which delivers about 90% less radiation than a standard X-ray, around ten times less. The scan takes just a few seconds and captures a single, joined-up picture of how your whole body lines up.
When MRI is the better choice: EOS shows bones, joints and alignment. If your clinician suspects a disc problem, a trapped nerve, or soft-tissue damage, MRI is usually the better test. The two work together. They do not compete.
If your back pain keeps coming back on one side and scans have not explained why, a standing alignment assessment may give you the answer.
Book a free video consultationWhat helps with lower back pain on one side?
The right approach depends entirely on what is causing the imbalance. Without knowing the pattern underneath, treatments tend to give only short-term relief. Here is the general picture.
- An accurate diagnosis first. Knowing whether the problem starts in your pelvis, legs or spine changes the direction of treatment completely.
- Targeted physiotherapy. Once the pattern is clear, a physiotherapist can strengthen the weaker side and release the overworked side, instead of treating both sides the same.
- Orthotics or heel raises. If a leg length difference is involved, a simple shoe insert may rebalance the load.
- Changing your daily habits. Avoiding long periods in one-sided postures (always crossing the same leg, carrying a bag on the same shoulder) can reduce flare-ups.
ScanAlign is a diagnostic imaging clinic. We do not provide treatment ourselves, but the detailed alignment report can help your physiotherapist, osteopath or consultant plan the right next step. If you would like to book a full-body standing scan, we start with a free video consultation to make sure it is right for you before you plan your trip to London.
Before you start a treatment, or change the one you are on, it is worth getting a detailed diagnosis. Knowing whether the cause sits in your pelvis, your legs or your spine changes what will actually help.
Frequently asked questions about back pain on one side
Back pain on the left side, sometimes called a left lower back strain, often points to an imbalance in your pelvis or a slight difference in leg length that shifts your weight to that side. The muscles and joints on the left end up working harder.
The mechanism is the same. Whether the pain is on the right or the left depends on which side is overloaded. The key question is why one side carries more stress, and that usually comes down to alignment.
Possibly. Kidney pain tends to sit higher, under the ribs, and may come with fever, nausea or changes in passing urine. If you have these symptoms, see your doctor. If the pain is lower and changes with movement, a musculoskeletal cause is more likely.
Yes. A lateral pelvic tilt raises one side of the pelvis, which forces the spine to bend sideways to make up for it. The muscles on the lower side often become overworked and painful.
No. ScanAlign is a private, self-pay clinic. You can refer yourself. We begin with a free video consultation to discuss your symptoms and confirm that a scan is right for you.
EOS uses about 90% less radiation than a standard X-ray, around ten times less. This makes it one of the lowest-dose imaging options available.
A standard X-ray can show bones and joints, but it usually focuses on one area and is often taken lying down. It may miss the wider alignment picture that explains why one side is overloaded.
The scan itself takes just a few seconds. The full appointment, including positioning and explanation, is longer, but the actual imaging is very quick.
It shows the full chain from your feet to your head in one image, while you carry your body weight. You can see whether the pelvis is tilted, whether the legs are different lengths, and how the spine makes up for it: all the things that explain why one side hurts.
For patients travelling from abroad, the price is confirmed during the free video consultation.
Still wondering why one side keeps hurting?
A free video consultation lets you talk through your symptoms from home and find out whether a standing alignment scan in London could help.
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- NHS. "Back pain." nhs.uk. Accessed 2025.
- Illés T, Somoskeöy S. "The EOS imaging system and its uses in daily orthopaedic practice." International Orthopaedics, 2012;36(7):1405-1411.
- Khamis S, Yizhar Z. "Effect of feet hyperpronation on pelvic alignment in a standing position." Gait & Posture, 2007;25(1):127-134.
Written by Abbas Dhami (Specialist Diagnostic Radiographer)
This page is general information and not a diagnosis.
ScanAlign is the trading name of CoreMed Solutions Ltd and operates under The Harley Street Hospital's CQC licence.