A toxicologist expert witness is someone with experience in the field of toxicology who is asked to testify in a legal case. The testimony is designed to provide the judge and jury with more information about the toxicological aspects of a case, with the understanding that most people not in the field of toxicology may have difficulty understanding such evidence and that certain details may be especially important when weighing the facts of a case. As with other expert witnesses, both sides may retain toxicology experts to testify and both sides may cross examine witnesses called by the other side.
In order to serve as a toxicologist expert witness, someone needs to have qualifications and experience in toxicology. Professors of toxicology and experienced forensic toxicologists with multiple publications to their names are often chosen for this type of testimony. Once on the stand, the witness may be asked to present credentials, and for this reason lawyers prefer to call people with credentials which will impress the court and reinforce the expert's position as a reliable source of information.
On the stand, a toxicologist expert witness can talk about techniques utilized to investigate evidence, can provide professional opinions on results and interpretations of such evidence, and talk about professional standards for people who work with such evidence. The witness may be used to support or undermine evidence which has been presented in a case. For example, if a defense attorney wants to argue that evidence was not processed properly and thus is not valid, a toxicologist expert witness could be asked to critique the processing procedures used in the forensic lab.
When someone is called as an expert witness, she or he collects a fee to compensate for time spent in court. This fee can vary, as in addition to giving the court time and energy, the witness is also providing a professional service. The fees paid to witnesses are part of the overall court costs involved in the case. In cases where the loser pays the court costs, someone who loses a case would be required to compensate expert witnesses called by the other side.
Such witnesses may also bring along visual aids to serve as props during their testimony, by arrangement with the court. For instance, a toxicologist expert witness might want to be able to visually illustrate the structure of various chemical compounds to provide people with visual references which will help them contextualize oral testimony. There may also be cases in which such witnesses provide evidence by deposition, rather than testifying in court.