One of the most common side effects after a lumbar puncture (LP) is headache. It is thought to be caused by irritation of the meninges. Removal of CSF from the subarachnoid space reduces the pressure of the CSF on the brain, which reduces intracranial pressure (not an increase). Removal of a standard amount of CSF during an LP does not cause severely elevated ICP. The nurse should not promise that the headache will resolve in a specific amount of time. Sometimes, the headache will be prolonged or refractory.