Sciatic nerve pain is most often caused by a herniated disc in the lower back pressing on one of the nerve roots that feed into the sciatic nerve. But herniated discs aren’t the only culprit. Bone spurs, narrowing of the spinal canal, vertebral slippage, and even a tight muscle deep in the hip can all irritate or compress the nerve and send pain shooting down your leg.
How the Sciatic Nerve Works
The sciatic nerve is the longest and thickest nerve in the body. It forms from five spinal nerve roots in the lower back and pelvis: L4, L5, S1, S2, and S3. These roots exit the spine, merge together in the pelvis, and travel down through each buttock and leg. Because the nerve originates from so many spinal levels, a problem at any one of those levels can trigger pain that radiates far from the actual source.
That radiating pattern is key to understanding sciatica. The injury or irritation almost always happens “upstream,” at or near the spine, but you feel it downstream as burning, shooting, or electric pain through the buttock and leg. Numbness or tingling in the foot is common too. In most cases, only one side is affected.
Herniated Discs: The Most Common Cause
Between each pair of vertebrae sits a rubbery disc with a tough outer shell and a softer gel-like center. A herniated disc happens when part of that soft center pushes through a crack in the outer shell. If the bulging material lands against a nerve root in the lumbar spine, it causes both direct pressure and a chemical inflammatory reaction around the nerve. That combination of compression and inflammation is what produces the intense pain, and sometimes numbness or weakness, that defines sciatica.
Not every herniated disc causes symptoms. Many people have disc bulges on imaging scans without ever feeling pain. Sciatica develops specifically when the herniation is large enough or positioned in a way that it contacts a nerve root.
Spinal Stenosis
Lumbar spinal stenosis is a gradual narrowing of the spinal canal in the lower back. As the canal shrinks, it squeezes the spinal cord or the nerve roots branching off from it. The narrowing is usually driven by age-related changes: thickening of ligaments, enlargement of the joints between vertebrae, and the growth of bone spurs that encroach on the space where nerves travel.
Stenosis tends to develop slowly, so symptoms often creep in over months or years rather than appearing suddenly. A hallmark pattern is pain that worsens when you stand or walk and eases when you sit down or lean forward, because bending slightly opens up the spinal canal and relieves some of the pressure on the nerves.
Spondylolisthesis
In spondylolisthesis, one vertebra slides forward over the one below it. That shift can pinch the nerve roots exiting at that level. There are two main types. In younger people, it often results from a stress fracture in a specific part of the vertebra (called isthmic spondylolisthesis). In older adults, it typically develops from age-related wear on the joints and discs (degenerative spondylolisthesis).
Most cases are low-grade, meaning the vertebra has slipped only a small amount. Even a modest amount of slippage, though, can be enough to compress a nerve root and trigger sciatic pain, especially during activities that extend or arch the lower back.
Piriformis Syndrome
Not all sciatic nerve pain starts in the spine. The piriformis is a small muscle deep in the buttock that runs from the lower spine to the top of the thighbone. The sciatic nerve passes directly beneath it, and in some people, right through it. When the piriformis muscle tightens, spasms, or swells from overuse or injury, it can press on the sciatic nerve and mimic the symptoms of spine-related sciatica.
Piriformis syndrome is relatively uncommon. Estimates suggest it accounts for roughly 0.3% to 6% of lower back and leg pain cases. Diagnosing it can be tricky because there’s no single definitive test. Providers typically diagnose it by ruling out spinal causes first and reproducing pain through specific hip stretches and rotations during a physical exam.
Bone Spurs and Other Overgrowths
As the spine ages, the body sometimes responds to wear and instability by growing extra bone along the edges of vertebrae. These bony overgrowths, called bone spurs or osteophytes, can narrow the openings where nerve roots exit the spine. When a spur forms in just the wrong spot, it puts steady pressure on the nerve root. Unlike a herniated disc, which can sometimes shrink over time as the body reabsorbs the material, bone spurs are permanent structures that tend to grow slowly larger.
Pregnancy and Sciatica
Sciatica during pregnancy has several overlapping causes. The hormone relaxin, which the body produces to loosen joints and ligaments in preparation for delivery, can create instability in the pelvis and lower spine. At the same time, as the belly grows, the spine’s curve becomes more pronounced to redistribute weight and maintain balance. Both changes can shift pressure onto the sciatic nerve roots.
During the second and third trimesters, the position of the fetus itself can compress the nerve even further. For most people, pregnancy-related sciatica resolves after delivery once the mechanical and hormonal triggers disappear.
Lifestyle Factors That Raise Your Risk
Certain habits and body composition factors make sciatica more likely. A large study published in The American Journal of Medicine found that obesity increased the risk of hospitalization for sciatica by 36%, and abdominal obesity specifically (measured by waist circumference rather than overall weight) raised the risk by 41%. Extra weight around the midsection increases the mechanical load on the lumbar spine and can accelerate disc degeneration.
Smoking was also significant. Current smokers had a 33% higher risk of being hospitalized for sciatica compared to nonsmokers. Smoking reduces blood flow to spinal discs, which accelerates their breakdown and makes herniation more likely. It also impairs the body’s ability to heal damaged tissue.
Prolonged sitting, heavy lifting, and repetitive twisting motions at work all place added stress on the lower back. Jobs that combine long periods of sitting with occasional heavy lifting seem to carry the highest risk, because sedentary muscles offer less support for the spine when sudden demands are placed on it.
Less Common Causes
In rare cases, sciatic pain originates from something other than a structural spine problem. Tumors, both benign and malignant, can grow along the nerve or in nearby tissue and mimic classic sciatica. Schwannomas, which are benign growths on the nerve’s outer sheath, are the most common nerve tumor but are still rare in the sciatic nerve specifically. Other documented causes include pelvic masses, infections near the spine, and inflammatory conditions like ankylosing spondylitis or sacroiliitis.
These uncommon causes are worth considering when sciatic pain doesn’t follow the typical pattern, doesn’t respond to standard treatment, or is accompanied by unexplained weight loss, fever, or progressive weakness.
When Sciatica Becomes an Emergency
A condition called cauda equina syndrome occurs when multiple nerve roots at the base of the spinal cord become severely compressed at the same time. It is rare but requires immediate surgery to prevent permanent damage. The warning signs are distinct from ordinary sciatica:
- Loss of bladder control: the bladder fills but you don’t feel the urge to urinate, or you experience sudden incontinence
- Loss of bowel control: inability to control stool
- Saddle numbness: loss of sensation in the groin, inner thighs, or buttock area
- Rapidly worsening weakness in one or both legs
- Sexual dysfunction with sudden onset
Any combination of these symptoms alongside back or leg pain requires evaluation by a neurosurgeon or spine surgeon as quickly as possible. The timing of surgical decompression directly affects the likelihood of recovering full nerve function.