A Spinal Tap is a somewhat dangerous medical procedure where a needle is inserted into the spine and spinal fluid is withdrawn. This is only done when it is an absolute medical necessity, because of the danger of damaging the spine.

It is useful in detection of certain diseases. There is an urban legend that a spinal tap can detect past LSD use. This turns out to not be true, and even if it was, spinal taps are far too dangerous to be used for drug testing.