Not long ago, I was at a cannabis expo and a gentleman came to our table to ask questions about some symptoms he was experiencing after using an oil extract.

He described a handful of symptoms, including confusion, loss of memory, headaches, and lack of coordination. I suspected that the product may have had residual solvents that hadn’t been fully removed during the processing.

Extract products have great benefits for patients. Cannabinoids are present in much higher concentrations, which means less product is required for a dose. Terpenes occur in higher concentrations in extracts as well. These high concentrations lend extracts to further processing, like being incorporated into edible products.

There are several ways to make extracts from dry flowers, including those that require large volumes of solvents. These large quantities improve the extraction efficiency and obtain higher levels of purity.

Once the extraction is complete, the solvents are removed from the product. However, problems can arise when this final stage is not executed properly and not all the residual solvents are removed.

When chemical solvents are not completely removed, they can be present in high enough concentrations to cause health problems for the patient, especially over time and with repeated exposure. Also, if cheaper solvents are used in place of high-quality ones, unwanted contaminants can create additional health risks.

Residual solvents can cause a wide variety of non-specific and sometimes irreversible health impacts to individuals exposed to them. They can cause eye, nose, and throat irritation, and they can affect the central nervous system, which can cause headaches, dizziness, and light-headedness progressing to unconsciousness, seizures, and possible death. A number of long-term adverse effects of solvents have been carcinogenic.

Testing for the absence of residual solvents is a critical step in ensuring that we have safe products for consumption.

Residual solvents are classified according to the health risk that they pose. Any discussion that you find about selecting residual solvents for extractions always includes a recommendation to use less toxic solvents. Solvents do not add any therapeutic benefit to the end user, and so should be removed from the product before consumption.

There are three categories of solvents, as described below. Generally, the residual solvents in Class 1 are so toxic they should be avoided, always.

Residual Solvent Class


Class 1

Solvents to be avoided Known human carcinogens Strongly suspected human carcinogens Environmental hazards

Class 2

Solvents to be limited

Non-genotoxic animal carcinogens, possible causative agents of other irreversible toxicity, such as neurotoxicity or teratogenicity

Solvents suspected of other significant but reversible toxicities

Class 3

Solvents with low toxic potential

Solvents with low toxic potential to humans; no health-based exposure limit is needed

They are less toxic in acute or short-term studies and negative in genotoxicity studies.

Some examples of the solvents are included in the table below. Class 3 solvents are less toxic and have lower risk to human health. There are no long-term toxicity or carcinogenicity studies for many of the Class 3 solvents.

Solvent Examples



Health / Safety Concern




Carbon Tetrachloride


Toxic, environmental hazard









Environmental hazard



Central nervous system: headaches, numbness, tremors



Central nervous system, neurotoxicity, can affect liver and kidneys



Probable carcinogen



Central nervous system effects: numbness, weakness, blurred vision



Headache, dizziness, insomnia, nausea, blindness



Depends on length and quantity of exposure



Depends on length and quantity of exposure



Depends on length and quantity of exposure



Depends on length and quantity of exposure

Health Canada does not specify a method that must be used for the extraction of cannabis oil. Producers can choose from a number of processes for extraction, including those that use carbon dioxide or water and those that are solvent-less.

These three have less potential for toxicity, but there are still concerns about the presence of residual solvents as solvents used to clean the equipment can find their way into the end product.

In Canada, the federal government is concerned about the health and safety of patients and have specified limits for residual solvents. Health Canada has mandated a requirement for residual solvent testing of all oil extracts.

In the lab, we use a robust method of analysing residual solvents called headspace sampling with a gas shromatograph. In a headspace analysis, the sample is heated in a gas-tight vial to vaporize the volatile components.

An automated syringe obtains a small sample of the gases in the headspace of a sealed vial from the sample preparation. The sample includes vapors of all volatile components in the vial. The sample is injected into a gas chromatograph for separation and detection of residual solvents.

This is an extremely sensitive test for most of the relevant volatile compounds. Residual solvent amounts are commonly expressed in parts per million (ppm).

With a good-quality test lab conducting the right testing, all adulterated material can be identified and destroyed to protect the end user.

I think about the gentleman that I met at the expo a lot. He was going to send me the remains of the oil that he had, in order for us to test it for the presence of residual solvents. I never heard back from him, and often wonder how he is making out. I hope he found the answers he needed to improve his health and begin to feel better.