Sciatica happens when something presses on or irritates the sciatic nerve, which runs from your lower back down through each leg. The lifetime incidence is estimated between 13% and 40% of the general population, making it one of the most common nerve pain conditions. The specific cause varies from person to person, but it almost always traces back to a problem in the lower spine or the soft tissues surrounding the nerve.
How the Sciatic Nerve Works
The sciatic nerve is about the diameter of your little finger, making it the largest nerve in your body. It’s formed by five nerve roots that exit your lower spine, travel through your pelvis, merge together, and run down the back of each leg. Because so many structures in your lower back sit close to these nerve roots, there are multiple points where something can go wrong.
Most cases of sciatica involve compression of the last lumbar nerve root (L5) or the first sacral nerve root (S1) right where they exit the spine. That’s why the pain typically shoots from the lower back into one leg, following the path of whichever nerve root is affected.
Herniated Discs: The Most Common Cause
A herniated disc in the lower back is the leading reason people develop sciatica, and it causes pain through two distinct mechanisms. The first is straightforward compression. When the soft inner material of a spinal disc bulges outward, it reduces the space in the spinal canal and physically squeezes the nerve root passing nearby. This is sometimes called mechanical radiculopathy.
The second mechanism is chemical. If the outer wall of the disc tears completely, the gel-like cushioning material inside can leak out and come into direct contact with the nerve root. This substance promotes inflammation because of its chemical composition. The resulting nerve irritation, called chemical radiculopathy, causes damage by triggering inflammation and disrupting normal nerve signaling. This is why some people with relatively small herniations still experience intense pain: even a small amount of leaked disc material can inflame a nerve root.
Spinal Stenosis and Age-Related Changes
As the spine ages, several changes can narrow the spaces where nerve roots travel. This narrowing, called spinal stenosis, is one of the most common causes of sciatica in people over 50.
Osteoarthritis breaks down cartilage between the vertebrae, and the body responds by growing extra bone. These bone spurs extend into the spinal canal, reducing the space available for nerves. At the same time, the discs between vertebrae dry out and flatten with age. A flattened disc is more likely to crack along its outer edge and bulge into the nerve’s path. The ligaments that hold the spine together also thicken over time due to arthritis, further crowding the canal. Spinal cysts, tumors, and old fractures can contribute as well, though these are less common.
The result is the same regardless of which structure is encroaching: the nerve root gets pinched in a tighter space, producing that characteristic shooting leg pain.
Vertebral Slippage
Spondylolisthesis occurs when one vertebra slides forward over the one below it. This shift in alignment puts direct pressure on nearby nerve roots and can narrow the openings where nerves exit the spine. It’s particularly common in the lower lumbar region, right where the nerve roots that form the sciatic nerve are most vulnerable. The slippage can result from a stress fracture in the vertebra (often from repetitive strain during adolescence), or from age-related degeneration of the joints and discs that normally keep the vertebrae locked in place.
Piriformis Syndrome
Not all sciatica originates in the spine. The piriformis is a small muscle deep in the buttock, and the sciatic nerve runs directly beneath it in most people. When this muscle tightens, spasms, or swells, it can compress the nerve and produce symptoms identical to spine-related sciatica.
Some people are more susceptible because of how their anatomy is wired. In the most common arrangement, the entire sciatic nerve passes underneath the piriformis. But in a notable percentage of the population, part of the nerve actually pierces through the muscle belly or exits above it. People with these variations are more likely to develop piriformis syndrome because the nerve is essentially threaded through the muscle rather than simply passing beneath it, making it more vulnerable to compression during movement or prolonged sitting.
Heavy Lifting and Workplace Risks
Incorrect lifting is one of the most reliable triggers for a new episode of sciatica. Bending at the waist to pick up something heavy puts enormous pressure on the lumbar discs, and that pressure can cause a disc to bulge, herniate, or shift out of alignment, compressing the sciatic nerve root. Jobs that involve repetitive heavy lifting carry a higher baseline risk.
Prolonged sitting is another major contributor, especially with poor posture. Slumping while seated increases the load on your lower lumbar discs and can gradually push disc material toward the nerve roots. People who spend long hours at a desk or behind the wheel are at particular risk. The combination of sustained pressure from sitting and occasional heavy lifting creates the conditions most likely to produce a disc problem over time.
Pregnancy and Hormonal Changes
Sciatica during pregnancy has both a mechanical and a hormonal explanation. As the uterus grows, the extra weight shifts your center of gravity forward, increasing the curve in your lower back. This posture change compresses the lumbar spine in ways it isn’t accustomed to.
At the same time, your body produces a hormone called relaxin, which loosens muscles, joints, and ligaments throughout your pelvis, back, and abdomen to prepare for delivery. While this flexibility is necessary, it also reduces spinal stability. Your back and pelvis become looser, your posture shifts further, and you’re more susceptible to the kind of disc shifts and nerve compression that cause sciatica. The growing uterus can also press directly on the sciatic nerve as it passes through the pelvis, adding a third source of irritation.
Diabetes and Nerve Damage
Chronically high blood sugar damages nerves through a less obvious route: it weakens the walls of the tiny blood vessels (capillaries) that supply nerves with oxygen and nutrients. Over time, this microvascular damage starves the nerve fibers, making them more vulnerable to injury and inflammation. While diabetic neuropathy most commonly affects the feet and hands first, it can involve larger nerves like the sciatic nerve as the condition progresses. People with poorly controlled diabetes who also have a mild disc bulge or spinal narrowing may develop sciatica more easily than they otherwise would, because their nerve is already compromised at the cellular level.
Warning Signs That Need Immediate Attention
Most sciatica, while painful, resolves over weeks to months. But certain symptoms indicate a serious complication called cauda equina syndrome, where the bundle of nerves at the base of the spine is severely compressed. These include sudden loss of bladder control or bowel control, numbness in the groin or inner thigh area (sometimes called saddle numbness), and rapidly worsening weakness in one or both legs. This is a medical emergency that requires same-day evaluation, because permanent nerve damage can result if pressure isn’t relieved quickly.