Poison is any substance that can harm or kill, and Toxicology is the study of it. The toxicity of a substance depends on the amount ingested, as well as the age, weight and state of health of the person who ingests it. Poison can be administered by ingestion, injection, or absorbed through the skin. Some poisons, such as snake venom, is only dangerous if it is absorbed in the bloodstream.

If poison is suspected at a crime scene the first step would be to take a sample from the victim, say from their bloodstream. As there would be other bodily fluids taken, the sample is treated to extract and purify the poison. Some poisons can be extracted using organic solvents, such as ether or chloroform. Chromatography is a technique that separates the various compounds in a sample, and is very useful when it comes to identifying poisons. Nowadays chromatography is coupled with a technique called mass spectrometry. Another way to identify poison is immunoassay. This is identifying antibodies made up for the foreign bodies in the blood, such as the poison. However this method is still being developed so is not widely used.

If a body suspected of poison has be exposed to nature and insects have been feeding off the body, there may be no or few soft tissues left. If the insects are still at the scene we can dissect them and extract the ingested tissues from their body. The poison may then be extracted from this.