Erowid’s DrugsData lab has recently changed from reporting simply “levamisole” to instead reporting “tetramisole/levamisole”. Tetramisole is the “real” primary name for this substance, a chemical that has two stereoisomers/enantiomers. For those unfamiliar, left (“lev”) and right (“dex”) isomers are only different in the same way that left- and right-handed gloves are different. They are also referred to as the “S-enantiomer” (lev) and the “R-enantiomer” (dex) of tetramisole. (Tetramisole refers to the “racemic” or mix of the S-enantiomer and R-enantiomer of the chemical.)
Many (though certainly not all) chemicals have this type of physical isomerism. An update in 2020 to the SWGDrug library that our lab uses as one of its sources changed the way it reports the name of the substance upon match. Neither the chief chemist at our lab nor any of our team remembers ever seeing the name “tetramisole” before December 2020. The technical distinction and update merits some further explanation.
The technical language can be pretty confusing. In some cases, the FDA allows commercial pharmaceuticals to use a shorthand, where they prepend “es” or “ar” (or “lev” and “dex”) to the front of a pharmaceutical name to denote an enantiomer-specific product. Examples include “armodafinil”, “eszopiclone”, and “escitalopram”.
Erowid’s DrugsData lab, using GC/MS testing, has no ability to isolate, separate, or identify which enantiomer is present, yet we’ve previously reported “levamisole” (the left or S-enantiomer) since first identifying this substance in cocaine samples in 2009 (see DrugsData Levamisole Results). Our lab’s techniques can’t distinguish the stereoisomer composition of any substance we analyze, e.g. a specific chemical identified in a single sample could be a 50/50 mix of R-enantiomer & S-enantiomer; 100% R-enantiomer and 0% S-enantiomer; 0% R-enantiomer and 100% S-enantiomer; or any ratio combination of the different physical isomers. This would be true for any lab using GC/MS for its testing process.
While DrugsData has been reporting “levamisole” since 2009, the question of enantiomers had not come up before. However, the “lev-” syllable should have been a giveaway. Sorry we didn’t identify this issue earlier!
In the last few years, research appears to indicate that most of the “levamisole” mixed with cocaine is actually the racemic tetramisole. In 2019, Madry et al. seemed to nail this issue down through analysis of hair samples from 627 cocaine users. The authors write: “Samples mainly contained racemic tetramisole (87.5%), only one sample contained levamisole only and two samples contained non-racemic [tetramisole].”
All of that, combined with our inability to test which spatial isomer we have in the samples we analyze, means that we’re going to switch to using “tetramisole” as the primary name for this substance, with “levamisole” being included so as to help avoid confusion due to the switch.
Reference:
Madry MM, Kraemer T, Baumgartner MR. “Cocaine adulteration with the anthelminthic tetramisole (levamisole/dexamisole): Long-term monitoring of its intake by chiral LC-MS/MS analysis of cocaine-positive hair samples”. Drug Test Anal. 2019 Mar;11(3):472-478. doi: 10.1002/dta.2505. Epub 2018 Oct 17. PMID: 30239147. Erowid Ref9448