Forensic chemical testing, including toxicology (for example ethanol in blood or a toxic substances in urine), would be accredited to the ISO 17025 standard for testing laboratories; under a Forensic accreditation program. Note that countries may also have specific requirements for Legal forensics testing, besides ISO 17025.
ISO 15189 Medical laboratories - Requirements for quality and competence is the international standard for medical laboratories. i.e. if the testing is a human medical pathology test, for example blood chemistry or microbiology, then ISO 17025 is typically not applicable. It is worth noting that ISO 15189 was developed based on ISO 9001 and ISO 17025. The requirements are similar, in the context of the type of testing and medical diagnostic risk. I suggest you look at the requirements of the accreditation programme provided by your accreditation body.