Are adverse effects of cannabidiol (CBD) products caused by tetrahydrocannabinol (THC) contamination?

CBD degradation

To investigate CBD degradation into ∆⁹-THC under acidic conditions, differently concentrated CBD in methanolic solutions was used in a range corresponding to typical amounts consumed with supplements based on commercial CBD (Supelco Cerilliant, certified reference material, #C-045, 1.0 mg/mL in methanol) supplied by Merck (Darmstadt, Germany). These solutions were exposed to an artificial gastric juice as well as different incubation times and stress factors such as storage under light and heat (see Table 1 for full experimental design). The solutions were stored either in standard freezer (-18°C) or refrigerator (8°C) or at room temperature (20°C). Increased temperatures were achieved using a thermostatically controlled laboratory drying oven type “UT6120” (Heraeus, Langenselbold, Germany) set to either 37°C or 60°C. The daylight condition was achieved by storage at a window (south side). For ultraviolet light exposure, six 25 W ultraviolet (UV) fluorescent tubes type “excellent E” (99.1% UVA) built into a facial tanner type “NT 446 U” (Dr. Kern GmbH, Mademühlen, Germany) were placed 15 cm from the surface of the solutions (open sample vials). In deviation of an experimental protocol of Merrick et al.²¹, a gastric juice without addition of surfactants was used, which was strictly produced according to the European pharmacopoeia²² (0.020 g NaCl + 0.032 g pepsin + 0.8 mL HCl (1 mol/L), filled up to 10 mL with water). As pure CBD was available only in methanolic solution, the final experimental setups contained 0.08 mol/L HCl and 1% methanol due to dilution (methanol residues in this order of magnitude are not interfering with the analysis).

To ensure the utmost analytical validity, all samples were independently measured on two different instruments, using a triple quadrupole mass spectrometer (TSQ Vantage, Thermo Fisher Scientific, San Jose, CA, USA) coupled with an LC system (1100 series, Agilent, Waldbronn, Germany) and also using a quadrupole time-of-flight (QTOF) mass spectrometer (X500, Sciex, Darmstadt, Germany) coupled with an UPLC system (1290 series, Agilent, Waldbronn, Germany). Both systems used the same type of separation column (Luna Omega Polar C18, 150 × 2.1 mm, 1.6 μm, 100 Å, Phenomenex, Aschaffenburg, Germany). The separation was isocratic with 25 % water (0.1 % formic acid) and 75 % acetonitrile (0.1 % formic acid) and a flow of 0.3 mL/min. In case of QTOF with 35 % water (0.1 % formic acid) and 65 % acetonitrile (0.1 % formic acid) and a flow of 0.45 mL/min. The evaluation took place after fragmentation of the mother ion into three mass traces for each compound. As quantifier for ∆⁹-THC and CBD, the mass transition m/z 315 to 193 was used. In case of QTOF, quantification was conducted over accurate mass and control of fragmentation pattern. CBD eluted as one of the first cannabinoids, a few minutes before ∆⁹-THC. As internal standards ∆⁹-THC-d₃ (Supelco Cerilliant #T-011, 1.0 mg/mL in methanol) was used for the quantification of ∆⁹-THC (Supelco Cerilliant #T-005, 1.0 mg/mL in methanol), and cannabidiol-d₃ (Supelco Cerilliant #C-084, 100 μg/mL in methanol) for quantification of CBD (Supelco Cerilliant #C-045, 1.0 mg/mL in methanol). The certified reference materials were obtained as solutions in ampoules of 1 mL, all supplied by Merck (Darmstadt, Germany). A limit of detection (LOD) of 5 ng/mL was determined. For both procedures, relative standard deviations better than 5% were achieved. Both methods are able to chromatographically separate ∆⁹-THC and CBD from their acids. Specificity was ensured using a certified reference material as a reference standard of THCA (Supelco Cerilliant #T-093, 1.0 mg/mL in acetonitrile). Baseline separation was achieved between ∆⁹-THC, ∆⁸-THC and THCA. Therefore, the reported values in this study are specific for ∆⁹-THC and CBD. In contrast to some previous studies based on gas chromatography, we do not report “total THC” or “total CBD”, which would be a sum of the free form and its acid.

∆⁹-THC contamination of commercial products

To study the possible influence of natively contained ∆⁹-THC in hemp products as a cause for adverse effects, a sampling of available CBD products registered as food supplement in the German State Baden-Württemberg, other available hemp extract products in retail, as well as all products available at the warehouse of a large internet retailer were sampled between December 2018 and December 2020. A total of 181 samples (see Table 2 for product designations) were analysed using the above-described liquid chromatographic method with tandem mass spectrometric detection (LC-MS/MS) for ∆⁹-THC content. For 2020 samples, the following parameters of the method were changed: separation column (Raptor, ARC-18, 150 × 2.1 mm, 2.7 μm, Restek, Bad Homburg, Germany). The separation was a gradient starting with 20% eluent A (0.1 % formic acid in water) and 80% eluent B (0.1 % formic acid in methanol) for 18 min, followed by 5% A and 95% B for 5 min, and back to 20% A and 80% B for 7 min. All methods were validated and externally accredited according to ISO 17025 standard. Recently, the method reported satisfactory results for ∆⁹-THC during the international government chemist CBD food and cosmetic ring rial²⁸.

For toxicological evaluation of the results, the lowest observed adverse effect level (LOAEL) of 2.5 mg ∆⁹-THC per day published by the European food safety authority (EFSA) based on human data (central nervous system effects and pulse increase) was used²⁷. Taking safety factors (factor 3 for extrapolation from LOAEL to no observed adverse effect level (NOAEL) and factor 10 for interindividual differences, total factor 30) into account, an acute reference dose (ARfD) of 1 μg ∆⁹-THC per kg body weight was derived²⁷. In their assessment, the Panel on Contaminants in the Food Chain of EFSA also considered interaction between ∆⁹-THC and CBD, but found the information controversial and not consistently antagonistic²⁷. This is consistent with more recent research of Solowij et al.²⁹ that the effects of ∆⁹-THC may even be enhanced by low-dose CBD (e.g., as found in food supplements) and may be particular prominent in infrequent cannabis users. However, the current scientific evidence does not allow for considering cumulative effects. The applicability of the acute reference dose (ARfD) of 1 μg ∆⁹-THC per kg body weight was re-confirmed by EFSA in 2020³⁰ and by the German Federal Institute for Risk Assessment (BfR) in 2021³¹. The BfR has also concluded that the previously suggested German guidance values, which had been considered in versions 1–3 of this article, no longer correspond to current scientific knowledge³¹. For this reason, the guidance values were removed from our assessment, which is now exclusively based on EFSA’s suggestions. For further details on interpretation of results and toxicity assessment, see Lachenmeier et al.². A detailed rationale for the estimation of the daily dose of products to be applied for the risk assessment has been provided in a correspondence article²⁴.

Direct pharmacological effect of CBD as explanation of adverse effects

There is not much evidence to assume that chemically pure CBD may exhibit ∆⁹-THC-like adverse effects. The World Health Organization (WHO) judged the compound as being well tolerated with a good safety profile³. Similar conclusions were made in a recent systematic review of CBD human trials³².

CBD doses in the food supplements on the market are typically much lower than the ones tested in clinical studies. Additionally, there is a 90-day experiment in rats with a hemp extract (consisting of 26% cannabinoids, out of which 96% were CBD and less than 1% ∆⁹-THC) from which a NOAEL of 100 mg/kg bw/day could be derived³³. Based on 100 mg/kg bw/day × 26% × 96%, this would be about 25 mg/kg bw/day for CBD (or 1750 mg/day for a person with a body weight of 70 kg). This NOAEL would not typically be reached by the CBD dosages in food supplements.

On the other hand, there are still many uncertainties and contradictions remaining regarding cannabinoid safety studies³⁴. The metabolism of CBD is very complex. The main human metabolite is 7-carboxy-cannabidiol (7-COOH-CBD; ~90 % of all drug-related substances measured in the plasma), which may form a reactive acyl-glucuronide^35–37. The toxicological profile of the metabolites has not been systematically investigated³⁴.

CBD conversion into THC as explanation of adverse effects

Some, partly older, in vitro studies put up hypotheses about the conversion of CBD to ∆⁹-THC under acidic conditions such as in artificial gastric juice^21,38–40. If these proposals could be confirmed with in vivo data, consumers taking CBD orally could be exposed to such high ∆⁹-THC levels that the threshold for pharmacological action could be exceeded⁴¹. However, taking a closer look at these in vitro studies raises some doubts. If CBD was to be converted to ∆⁹-THC in vivo, typical ∆⁹-THC metabolites should be detectable in blood and urine, but this has not been observed in oral or inhalatory CBD studies^42–44. Due to the contradicting results, a replication of the in vitro study of Merrick et al.²¹ was conducted using an extended experimental design. A more selective LC-MS/MS method and also an ultra-high pressure liquid chromatographic method with quadrupole time-of-flight mass spectrometry (UPLC-QTOF) were used to investigate the CBD degradation.

Under these conditions in contrast to Merrick et al.²¹, no conversion of CBD to ∆⁹-THC was observed in any of the samples. Only in case of the positive control (2 week storage in 0.5 mol/L HCl and 50% methanol), a complete degradation of CBD into 27% ∆⁹-THC and other not identified products (with fragments similar to the ones found in cannabinol and ∆⁹-THC fragmentations but with other retention times) was observed (Table 1, underlying data²³). From an analytical viewpoint, the use of less selective and specific analytical methods, especially from the point of chromatographic separation, could result in a situation in which certain CBD degradation products might easily be confused with ∆⁹-THC due to structural similarities. Thus, similar fragmentation patterns and potentially overlapping peaks under certain chromatographic conditions might have led to false positive results in the previous studies. In conclusion of our degradation experiments, we agree with more recent literature^45–47 that CBD would not likely react to ∆⁹-THC under in vivo conditions. The only detectable influence leading to degradation is strong acidity, which should be avoided in CBD formulations to ensure stability of products⁴⁸. Similar observations were recently provided by Yangsud et al. determining CBD as stabile under stress conditions, other than acidic or alkaline conditions⁴⁹. Transformation of CBD may also occur in acidified plasma samples or during pyrolysis gas chromatography^50,51.

∆⁹-THC contamination as cause of adverse effects

Out of 181 samples, 21 samples (12% of the collective) have the potential to exceed the ∆⁹-THC LOAEL and were assessed as harmful to health. 82 samples (45% of the collective) were classified as unsuitable for human consumption due to exceeding the ARfD (see Table 2, underlying data²³). Furthermore, all food samples (i.e., all samples except CBD liquids intended to refill electronic cigarettes) have been classified as non-compliant to Regulation (EU) 2015/2283 of the European Parliament and of the Council of 25 November 2015 on novel foods⁵² and therefore being unauthorized novel foods⁵³. The labelling of all food samples was also non-compliant to Regulation (EU) No 1169/2011 of the European Parliament and of the Council of 25 October 2011 on the provision of food information to consumers⁵⁴, e.g. due to lack of mandatory food information such as ingredients list or use of unapproved health claims in accordance to Regulation (EC) No 1924/2006 of the European Parliament and of the Council of 20 December 2006 on nutrition and health claims made on foods⁵⁵. In summary, none of the food products in our survey was found as being fully compliant with European food regulations.

The ∆⁹-THC dose leading to intoxication is considered to be in the range of 10 to 20 mg (very high dose in heavy episodic cannabis users up to 60 mg) for cannabis smoking⁵⁶. The resorption of orally ingested ∆⁹-THC varies greatly inter-individually with respect to both total amount and resorption rate⁵⁷. This might be one of the reasons for the individually very different observed psychotropic effects. A single oral dose of 20 mg THC resulted in symptoms such as tachycardia, conjunctival irritation, “high sensation” or dysphoria in adults within one to four hours. In one out of five adults, a single dose of 5 mg already showed corresponding symptoms⁵⁸.

Some of the CBD oil supplements contained ∆⁹-THC in doses up to 30 mg (in this case in the whole bottle of 10 ml), which can easily explain the adverse effects observed by some consumers. Interestingly, it was observed that the symptoms reported with cannabidiol exposures in the so far largest epidemiological study²⁰ were ∆⁹-THC-like symptoms⁵⁹.

Most of the CBD oils with dosage of around 1 mg ∆⁹-THC per serving offer the possibility to achieve intoxicating and psychotropic effects due to this compound if the products are used off-label (i.e. increase of the labelled maximum recommended daily dose by factors of 3–5, which is probably not an unlikely scenario. Some manufacturers even suggest an increase of daily dosage over time). Generally, these products pose a risk to human health considering EFSA’s ARfD that is considerably exceeded, even without consideration of THCA.

Hence our results provide compelling evidence that THC natively contained in CBD products may be a direct cause for adverse effects of these products. Obviously, there seems to be an involuntary or deliberate lack of quality control of CBD products. Claims of “THC-free”, used by most manufacturers, even on highly contaminated products – sometimes based on the use of unsuitable analytical methodologies with limits of detection in the percentage range –, have to be treated as fraudulent or deceptive food information.

Underlying data

Open Science Framework: Dataset for “Are adverse effects of cannabidiol (CBD) products caused by delta9-tetrahydrocannabinol (THC) contamination?” (Version 3) https://doi.org/10.17605/OSF.IO/F7ZXY²³

This project contains the following underlying data:

Data are available under the terms of the Creative Commons Zero “No rights reserved” data waiver (CC0 1.0 Public domain dedication).