Forensic Toxicology: The Dose Makes the Poison

by Carolina Staff

Forensic toxicology is the analysis of biological samples for the presence of drugs, chemicals, or other toxins. Forensic toxicologists look specifically at substances that may cause death, impairment of human performance, or enhancement of human performance. They also look at how the presence of certain substances can affect compliance with workplace policies or federal and state laws.

According to the National Forensic Science Technology Center, there are 3 objectives for using forensic toxicology in an investigation:

  1. Establish if toxicants are present and capable of contributing to death
  2. Establish if toxicants are present and capable of causing behavioral changes
  3. Establish if substances are present and whether or not they represent legitimate use or exposure, such as prescribed medications or workplace exposures

What kinds of samples are used in toxicology testing?

Forensic toxicologists use a number of bodily fluids and tissues for analysis. Blood, urine, hair, fingernails, and air in the lungs can be obtained from living specimens. Additionally, the vitreous humor, liver, gastric contents, bile, lung fluid, and kidneys can be analyzed from post-mortem subjects. Blood is the preferred specimen for detecting and quantifying drugs and other toxicants. However, urine is the most common sample used for drug testing in the workplace. The presence of drugs or toxicants in urine indicates that a substance has been in the blood recently, possibly within the last few days. Different toxicants have different metabolic pathways, so they appear in bodily fluids at different intervals and persist for different lengths of time.

Routine toxicology may include testing for alcohol, analgesics, antidepressants, antihistamines, antipsychotics, benzodiazepines, cannabis, cocaine, narcotic analgesics, and stimulants. As with all other forensic evidence, chain of custody dictates that evidence for toxicological analysis must be collected and placed in tamper-proof bags with numbered seals. 

How are the samples analyzed?

Forensic toxicologists use a variety of analytic techniques that range from urine test strips to computerized mass spectroscopy. Test strips, commonly called dipsticks, are actually immunoassays and are the most common drug screening technique. Immunoassays use antibodies that react with targeted substances found in the urine as drugs are metabolized in the body. Binding of such antibodies to specific targets produces a color change that can be visualized when a threshold limit is met. Immunoassays are used as an initial drug screening, indicating simply whether the targeted substance is present or not. The immunoassay does not identify a specific quantity of a particular drug, just presence or absence in the urine. For immunoassays that yield a positive result, further analysis is required.

A drug test comes back positive–what happens next?

Two common analytic techniques used to confirm positive immunoassay results are chromatography and mass spectroscopy. High performance liquid chromatography (HPLC) and gas chromatography (GC) separate compounds based on different strengths of attraction between the compounds and a stationary phase. The stationary phase coats the walls of very small tubes. The greater the attraction between a substance and the stationary phase, the longer the substance remains in the column. The length of time a substance spends in the column is called retention time. This value helps forensic toxicologists separate and categorize components in a sample. They use a mass spectrometer (MS) to identify the separated components. Mass spectrometers identify components based on fragmentation patterns. Toxicologists then cross-reference the patterns against known chemicals for a match.  

Related items

Introduction to Toxicology Kit
Drugs and Poisons Analysis Kit
Forensic Chemistry: Drug Detection Kit


Bacher, Dr. Alfred D. (2016, April 01). Gas Chromatography Theory. Retrieved from

National Forensic Science Technology Center. (2013). A Simplified Guide to Forensic Toxicology. Retrieved from

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