Sciatica (or sciatic neuritis) is a set of symptoms including pain that may be caused by general compression and/or irritation of one of five spinal nerve roots that give rise to each sciatic nerve, or by compression or irritation of the left or right or both sciatic nerves. The pain is felt in the lower back, buttock, and/or various parts of the leg and foot. In addition to pain, which is sometimes severe, there may be numbness, muscular weakness, pins and needles or tingling and difficulty in moving or controlling the leg. Typically, the symptoms are only felt on one side of the body.

Although sciatica is a relatively common form of low back pain and leg pain, the true meaning of the term is often misunderstood. Sciatica is a set of symptoms rather than a diagnosis for what is irritating the root of the nerve, causing the pain. This point is important, because treatment for sciatica or sciatic symptoms will often be different, depending upon the underlying cause of the symptoms.

Sciatica is generally caused by the compression of lumbar nerves L4 or L5 or sacral nerves S1, S2 or S3, or far less commonly, by compression of the sciatic nerve itself. When sciatica is caused by compression of a dorsal nerve root (radix) it is considered a lumbar radiculopathy (or radiculitis when accompanied with an inflammatory response) from a spinal disc herniation (a herniated intervertebral disc in the spine), or from roughening, enlarging, and/or misaligning of the vertebrae (spondylolisthesis), or degenerated discs. Sciatica due to compression of a nerve root is one of the most common forms of radiculopathy.

Pseudosciatica or non-discogenic sciatica, which causes symptoms similar to spinal nerve root compression, is most often referred pain from damage to facet joints in the lower back and is felt as pain in the lower back and posterior upper legs. Pseudosciatic pain can also in rare cases be caused by compression of peripheral sections of the nerve, usually from soft tissue tension in the piriformis or related muscles.

One of the possible causes of sciatica is a spinal disc herniation pressing on one of the sciatic nerve roots. The spinal discs are composed of a tough spongiform ring of cartilage (annulus fibrosus) with a more malleable center (nucleus pulposis). The discs separate the vertebrae, thereby allowing room for the nerve roots to properly exit through the spaces between the L4, L5, and sacral vertebrae. The discs cushion the spine from compressive forces, but are weak to pressure applied during rotational movements. That is why a person who bends to one side, at a bad angle to pick something up, may more likely herniate a spinal disc than a person jumping from a ladder and landing on his or her feet.

Herniation of a disc occurs when the liquid center of the disc bulges outwards, tearing the external ring of fibers, extrudes into the spinal canal, and compresses a nerve root against the lamina or pedicle of a vertebra, thus causing sciatica. This extruded liquid from the nucleus pulposus may cause inflammation and swelling of surrounding tissue which may cause further compression of the nerve root in the confined space in the spinal canal.

Sciatica caused by pressure from a disc herniation and swelling of surrounding tissue can spontaneously subside if the tear in the disc heals and pulposis extrusion and inflammation cease.

Sciatica can be caused by tumours impinging on the spinal cord or the nerve roots. Severe back pain extending to the hips and feet, loss of bladder or bowel control, or muscle weakness, may result from spinal tumours. Trauma to the spine, such as from a car accident, may also lead to sciatica.

Other compressive spinal causes include spinal stenosis, a condition in which the spinal canal (the spaces through which the spinal cord runs) narrows and compresses the spinal cord, cauda equina, and/or sciatic nerve roots. This narrowing can be caused by bone spurs, spondylolisthesis, inflammation, or herniated disc which decreases available space for the spinal cord, thus pinching and irritating nerves from the spinal cord that travel to the sciatic nerves.

In 15% of the population, the sciatic nerve runs through the piriformis muscle rather than beneath it. When the muscle shortens or spasms due to trauma or overuse, it can compress or strangle the sciatic nerve beneath the muscle. Conditions of this type are generally referred to as entrapment neuropathies; in the particular case of sciatica and the piriformis muscle, this condition is known as piriformis syndrome. It has colloquially been referred to as "wallet sciatica" since a wallet carried in a rear hip pocket will compress the muscles of the buttocks and sciatic nerve when the bearer sits down. Piriformis syndrome may be a cause of sciatica when the nerve root is normal.

Another source of sciatic symptoms is active trigger points of the lower back and the gluteus muscles. In this case, the referred pain is not consequent to compression of the sciatic nerve, though the pain distribution down the buttocks and leg is similar. Trigger points occur when muscles become ischemic (having low blood flow) due to injury or chronic muscular contraction. The most commonly associated muscles with trigger points triggering sciatic symptoms are: the quadratus lumborum, the gluteus medius, the gluteus minimus, and the deep hip rotators.

Sciatica may also be experienced in pregnancy, primarily resulting from the uterus pressing on the sciatic nerve, and, secondarily, from the muscular tension and/or vertebral compression consequent to carrying the extra weight of the fetus, and the postural changes inherent to pregnancy.

The risk of self-inflicted sciatica has increased in recent years with, for instance, sitting on a wallet or standing for prolonged periods of time every day which can cause self-inflicted sciatica. Symptoms of numbness and/or pain behind the knee cap are associated with this form of sciatica. Work-related sciatica may be caused by the use of tool belts which hang around the hips and cause significant misalignment of the sacral vertebrae over long time periods.