Review: Influence of Thermal Hair Straightening on Cannabis and Cocaine Content in Hair

Emily C. Lennert





hair, drug, cannabis, cocaine, THC, cannabinol, cocaethylene, benzoylecgonine, gas chromatography, mass spectrometry, GC-MS

Article Reviewed

  1. Ettlinger, J.; Yegles, M. Influence of thermal hair straightening on cannabis and cocaine content in hair. Forensic Science International. 2016, 265, 13-16.


The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.


Certain cosmetic treatments, such as bleaching and dyeing hair, are known to influence the concentration of drugs observed in hair samples. Similarly, it must be considered whether thermal hair straightening has an effect on the drug content of hair samples. Hair straightening is a common treatment that is used often, if not daily, by many people. The goal of this study was to assess the influence of thermal hair straightening on the concentration of tetrahydrocannabinol (THC), cannabinol (CBN), cocaine, benzoylecgonine (BZE), and cocaethylene in hair samples. Cannabinol is a common metabolite of THC. Similarly, metabolism of cocaine will commonly generate BZE and cocaethylene.

Drug positive hair samples were obtained from samples that were gathered for driving license cases: 17 cannabis positive samples and 7 cocaine positive samples. Two strands were selected from each case sample. One strand was left untreated while the other was subjected to thermal treatment. In typical thermal hair straightening, metal plates are heated to 150-200 ˚C and passed over the hair strands two to three times, for about two seconds each pass. Daily use corresponds to approximately 2-3 minutes of contact between the hair strand and metal plates per month. Therefore, a contact time of 1 minute was selected (30 passes of 2 seconds each) to mimic daily use, with the curling iron set to 200 ˚C.

Cannabis Sample Preparation

Cannabis containing hair samples were washed with water and acetone prior to pulverization with a ball mill. Once the hair sample was powdered, it was incubated in sodium hydroxide (NaOH) for 30 min at 90 ˚C, followed by solid phase extraction and silylation, which is a process where silyl group is attached to the drug to increase the ability to analyze the drug on a gas chromatograph-mass spectrometer (GC-MS). Samples were then analyzed by GC-MS to determine the concentration of cannabis and its derivatives.

Cocaine Sample Preparation

Cocaine containing samples were washed with water and acetone, then pulverized with a ball mill and incubated in phosphate buffer for 2 hours in an ultrasonic bath. The sample was then subjected to solid phase extraction, then derivatization using pentafluoropropionic anhydride and pentafluoropropanol. Samples were then analyzed by GC-MS for concentration determination.

Cannabis Sample Results

Overall, a median decrease of 10.1% in THC concentration was observed between the untreated and the heat treated hair samples. A decrease in THC was observed in 11 of the 17 hair samples, ranging from 3-100% decrease. However, in 6 samples, an increase in THC concentration was observed, ranging 4.6-67.2%. In all samples, CBN was higher after thermal treatment, with the increase in concentration ranging from 72-751%, with a median value of 138%. The authors proposed that the decrease of THC and increase of CBN may be due to thermal degradation of THC in comparison to CBN, as reported by a previous study. The ratio of THC/CBN was examined. In untreated hair, the ratio was determined to be approximately 7.5; however, in the heat treated hair the ratio drops to 2.4. The authors propose that a THC/CBN ratio below 4 may be indicative that hair was thermally treated, but note that further studies will be required to confirm this.

THC results appeared to be dependent on hair color, as shown in Figure 2 within the study. Higher concentrations after treatment were observed in darkly colored hair. The authors propose that this is due to the physical characteristics of the hair, noting that lightly colored hair is generally thinner and therefore results in greater heat conversion of THC into CBN. Whereas, darkly colored hair may be thicker and less sensitive to heat, resulting in less conversion of THC to CBN.

Cocaine Sample Results

Thermal straightening showed an appreciable impact on the cocaine content of hair samples. In all hair samples, a decrease in cocaine concentration was observed, ranging in decrease from 44.6-100%, with a median decrease of 75.4%. Likewise, cocaethylene showed a decrease in 4 samples, median 52.1% decrease, and was not present in 3 samples. Conversely, BZE was observed to increase in all samples, ranging in increase from 3.3-48.2%, with a median increase of 20.5%. A BZE/cocaine ratio was determined for untreated and treated hair. Untreated hair resulted in a ratio of 0.6; whereas, treated hair resulted in a ratio of 1.6. The authors propose that a BZE/cocaine ratio higher than 1 may be indicative of thermal hair straightening, but note that further studies are required.

To determine the cause of the decrease in cocaine and increase in BZE, additional studies were conducted. 200 ng of cocaine was placed into 2 vials, one containing 100 μL of water and the other dry. These vials were placed in an oven at 200 ˚C for 1 minute, then rapidly cooled to simulate the experimental conditions of the hair. In the vial with water, cocaine decreased from 200 ng to 57 ng, and 65 ng of BZE was present. In the dry vial, cocaine decreased from 200 ng to 36 ng, and 2 ng of BZE was present. The authors suggest that these results are indicative of a heat conversion of cocaine into BZE, which may be due to hydrolysis due to the increased yield of BZE in the vial containing water.

Scientific Highlights

  • Whether THC increased or decreased was dependent on hair color: dark hair colors saw an increase in THC concentration; whereas, lightly colored hair showed a decrease in THC concentration.
  • CBN content increased after thermal treatment for all cannabis samples.
  • Cocaine decreased in all cocaine hair samples after thermal treatment.
  • BZE increased in all cocaine hair samples after thermal treatment. It is proposed that cocaine undergoes heat conversion to BZE.


Thermal hair straightening affects the observed concentration of drugs in hair samples, which may lead to misleading results if not accounted for in hair analysis.

Potential Conclusions

  • Thermal hair straightening should be documented when hair samples are taken for drug analysis.
  • Thermal hair straightening may have a significant effect on the observed concentrations of THC, CBN, cocaine, BZE, and cocaethylene in hair.