A lumbar puncture (LP), also known as a spinal tap, is a medical procedure in which a needle is inserted into the spine, most commonly to collect cerebrospinal fluid (CSF) for diagnostic testing. The main reason for a lumbar puncture is to help diagnose diseases of the central nervous system, including the brain and spine. Examples of these conditions include meningitis and subarachnoid hemorrhage. It may also be used therapeutically in some conditions. Increased intracranial pressure (pressure in the skull) is a contraindication, due to risk of brain matter being compressed and pushed toward the spine. Sometimes, lumbar puncture cannot be performed safely (for example a severe bleeding tendency). It is regarded as a safe procedure, but post-dural-puncture headache is a common side effect.