FORENSIC TOXICOLOGY.NETOkorie Okorocha, J.D., M.S., M.S.Forensic Toxicology Expert Witness

EVIDENCE LIBRARY

A Positive Methamphetamine Result: Source, Isomers, and Metabolic Precursors

By forensic toxicology working group members

Learn how methamphetamine isomers, inhalers, medications, metabolic precursors, and confirmatory methods affect interpretation of a positive result.

By Okorie Okorocha, JD, MS, MS

A laboratory report that identifies methamphetamine answers an important question: methamphetamine was detected in the tested specimen above the laboratory’s reporting threshold. It does not always answer a second question that may be central to the case: where did the methamphetamine come from?

Source interpretation may require stereoisomer testing, medication and product history, metabolite patterns, analytical details, and an understanding of drugs that can be metabolized to amphetamine or methamphetamine.

Methamphetamine has different stereoisomeric forms

Methamphetamine exists in d- and l-forms. They share the same molecular formula but differ in three-dimensional arrangement and pharmacological activity.

d-Methamphetamine is the form most strongly associated with central stimulant effects and illicit methamphetamine. l-Methamphetamine has been used as an ingredient in certain nonprescription nasal inhalers. A routine non-chiral confirmation may identify methamphetamine without separating these forms.

The analytical distinction matters because a result containing only l-methamphetamine can support a different source explanation from a result dominated by d-methamphetamine. The laboratory must actually perform an appropriate chiral or enantiomer-specific method before an expert can rely on that distinction.

Controlled inhaler research

Mendelson and colleagues studied 12 participants who used an l-methamphetamine nasal inhaler at recommended and higher defined exposures. They measured pharmacokinetic, cardiovascular, physiological, and subjective outcomes. Under the tested inhaler conditions, delivered doses were small and measured physiological effects were minimal.

Earlier research illustrates why the laboratory method matters. Fitzgerald and colleagues collected urine from three volunteers after repeated use of a nasal inhaler containing the R(-), or l-, form of methamphetamine. The specimens could produce methamphetamine-positive findings with several screening and confirmation procedures used at the time. Chiral derivatization separated the stereoisomers and supplied information about the probable source.

That 1988 experiment used an intensive exposure schedule and historical products and assays. It should not be treated as a concentration benchmark for every current inhaler user. Its durable lesson is that a non-chiral method can correctly identify methamphetamine while leaving the source unresolved.

The study should not be generalized to d-methamphetamine or illicit exposure. Its proper use is narrower: it documents a legitimate l-methamphetamine source and provides controlled information about that source.

It also does not mean that every methamphetamine-positive specimen came from an inhaler. Product history, isomer composition, concentrations, metabolite findings, timing, and analytical quality still have to fit.

Metabolic precursors can complicate interpretation

Cody reviewed fourteen drugs that can produce amphetamine or methamphetamine through metabolism. The group included pharmaceutical or historically used compounds such as benzphetamine, selegiline, famprofazone, clobenzorex, and several anorectic agents.

Benzphetamine

Budd and Jain reported that a single volunteer given benzphetamine excreted detectable amphetamine and methamphetamine even though unchanged benzphetamine was not detected in the serial urine specimens. Inoue and Suzuki later studied benzphetamine and furfenorex metabolism in healthy volunteers and identified amphetamine and methamphetamine as minor metabolites alongside more abundant metabolites.

These studies show why confirmation of methamphetamine does not automatically identify the administered parent drug. They do not establish that benzphetamine caused an individual result. The 1978 report involved one volunteer, the 1986 study used historical analytical methods, and a current evaluation should look for the parent medication, characteristic metabolites, dose history, isomer pattern, and expected ratios.

Famprofazone

Famprofazone is another historical analgesic precursor that can form methamphetamine and amphetamine. Oh, Hong, and Kang measured methamphetamine in plasma and urine after controlled famprofazone administration to six volunteers. Yoo, Chung, and Choi later described a case and a three-volunteer experiment in which famprofazone administration produced methamphetamine and amphetamine findings.

A 2009 conference abstract by Chan and colleagues reported different methamphetamine and amphetamine enantiomer proportions in urine from six famprofazone recipients and 12 samples described as coming from methamphetamine users. Because the item is an abstract rather than a complete peer-reviewed report, its methods and results cannot receive the same weight as a full article.

Famprofazone availability is jurisdiction- and time-dependent. A defensible source opinion requires evidence that the product was actually available and taken, followed by testing capable of evaluating the predicted metabolites and stereoisomer pattern.

This does not create a generic defense for every positive result. Some compounds may not be available or approved in the relevant location, and some produce characteristic metabolite or isomer patterns. The review instead supplies a structured question: could an identified medication or precursor plausibly generate the reported analytes, in the reported forms, under the facts of this case?

Screening cross-reactivity is a separate issue

A presumptive amphetamine-class immunoassay can react with structurally related compounds even when confirmatory testing does not identify amphetamine or methamphetamine. That is cross-reactivity.

Metabolic production is different. In that situation, the body actually forms an analyte that a specific confirmatory method may correctly identify. Isomer composition is a third issue: the laboratory may correctly identify methamphetamine but leave its stereochemical form unresolved.

These questions should not be collapsed:

  • Did the screening antibody respond to another compound?
  • Did confirmation specifically identify methamphetamine and amphetamine?
  • Did another administered drug metabolize to those analytes?
  • What were the d- and l-isomer proportions?
  • Does the proposed source fit the complete metabolite pattern and timeline?

Why the amphetamine-to-methamphetamine relationship matters

Amphetamine can be a metabolite of methamphetamine, but amphetamine also has independent sources. The presence or absence of amphetamine, its concentration relative to methamphetamine, urine pH, collection time, and reporting thresholds may provide context.

No single ratio should be treated as universal. Metabolism, dose pattern, specimen type, timing, isomer composition, and individual variation all affect the observed values.

Records needed for a source opinion

A scientifically bounded review should obtain:

  • the original screening and confirmation records;
  • the exact confirmed analytes, concentrations, and reporting cutoffs;
  • chromatograms, ion ratios or transitions, calibrators, and quality controls;
  • whether the method was chiral and the reported d/l proportions;
  • complete prescription, over-the-counter, supplement, and foreign-medication history;
  • product name, formulation, lot, dosing, and timing when a legitimate source is asserted;
  • urine pH, creatinine, specific gravity, and collection timeline;
  • amphetamine and other relevant metabolites; and
  • chain-of-custody and specimen-identity documentation.

The proper conclusion may be that illicit d-methamphetamine use is supported, that a legitimate l-methamphetamine or precursor source is supported, or that the available testing cannot resolve the source. The science should determine which conclusion is justified.

References

  • Mendelson JE, McGlothlin D, Harris DS, et al. The clinical pharmacology of intranasal l-methamphetamine. BMC Clinical Pharmacology. 2008;8:4. https://doi.org/10.1186/1472-6904-8-4
  • Cody JT. Metabolic precursors to amphetamine and methamphetamine. Forensic Science Review. 1993;5(2):109-127. https://pubmed.ncbi.nlm.nih.gov/26270078/
  • Smith ML, Nichols DC, Underwood P, et al. Methamphetamine and amphetamine isomer concentrations in human urine following controlled Vicks VapoInhaler administration. Journal of Analytical Toxicology. 2014;38(8):524-527. https://doi.org/10.1093/jat/bku077
  • Cody JT, Schwarzhoff R. Interpretation of methamphetamine and amphetamine enantiomer data. Journal of Analytical Toxicology. 1993;17(6):321-326. https://doi.org/10.1093/jat/17.6.321
  • Budd RD, Jain NC. Metabolism and excretion of benzphetamine: sources of error in reporting results. Journal of Analytical Toxicology. 1978;2(6):241.
  • Inoue T, Suzuki S. The metabolism of benzphetamine and furfenorex in man. Xenobiotica. 1986;16(7):691-698. https://doi.org/10.3109/00498258609043559
  • Fitzgerald RL, Ramos JM Jr, Bogema SC, Poklis A. Resolution of methamphetamine stereoisomers in urine drug testing: urinary excretion of R(-)-methamphetamine following use of nasal inhalers. Journal of Analytical Toxicology. 1988;12(5):255-259. https://doi.org/10.1093/jat/12.5.255
  • Oh ES, Hong SK, Kang GI. Plasma and urinary concentrations of methamphetamine after oral administration of famprofazone to man. Xenobiotica. 1992;22(3):377-384. https://doi.org/10.3109/00498259209046649
  • Yoo Y, Chung H, Choi H. Urinary methamphetamine concentration following famprofazone administration. Journal of Analytical Toxicology. 1994;18(5):265-268. https://doi.org/10.1093/jat/18.5.265

Educational information only. It is not a case-specific toxicology opinion or legal advice.