Drug concentration is the quantity of a drug present in a biological sample. Measuring drug amounts in biological samples allows the clinician to understand how a drug is absorbed, distributed, metabolized, and excreted. Samples can be obtained through invasive or non-invasive methods. Invasive techniques involve surgical or parenteral interventions to gather blood, cerebrospinal fluid, or tissue biopsy. Conversely, non-invasive approaches provide samples like urine, feces, and saliva.

Plasma — a component of blood — is commonly used for pharmacokinetic studies. The total plasma concentration comprises unbound drugs and those bound to proteins in circulation. The filtered plasma's unbound drug concentration signifies the target site's concentration.

Tissue biopsies confirm if a drug has reached the target tissue at the necessary concentration. Urine analysis indirectly verifies the drug's bioavailability, reflecting the systemic absorption rate. Fecal measurements reveal unabsorbed oral doses or drugs expelled via bile.

In forensics, examining drug levels in autopsy tissues or bodily fluids can provide potential causes of death, such as overdose, poisoning, or drug abuse. As a result, measuring drug concentration is crucial for clinical and forensic applications, offering significant insights into drug efficacy, safety, and misuse.