Using blood for drug testing or medication monitoring provides invaluable information on compliance with medication regimens, court mandates, substance use treatment, and other directives. When it becomes pertinent to know how much of a substance is present, how that quantity compares to a person’s dosing regimen, and/or how the individual is metabolizing a prescribed medication, then it may be time to use blood drug testing. Blood has a shorter window of detection than urine, but it can provide substantial insights into recent medication use and illicit drug use.