CBD | Science

Common CBD Oil Side Effects And Risks

While Cannabidiol (CBD) might be somewhat new to most of us, it’s been around since the 1970s.

Dr Abraham Benavides


Dr Abraham Benavides

Medical Doctor

Initially pushed aside in research in favour of medical marijuana research examining its well-known sibling tetrahydrocannabinol, known as THC for short. CBD is back on the research agenda to monitor its effectiveness, potential benefits and associated CBD oil side effects.

In the last three years, the CBD oil market has moved from a small niche industry directly into the mainstream, boosted by the US federal legalisation of hemp in late 2018 and recent shifts in consumer preferences favouring natural supplements and products.

In this post, we are going to walk through some of the history behind why a bank of clinical evidence is lacking and discuss the most relevant research studies, for you to decide if the potential benefits balance the side effects of CBD oil.

CBD Oil Side Effects Illustration

CBD Is Well Tolerated

While CBD is well-tolerated and considered safe, like any supplement we consume it has the potential to yield side effects which can manifest themselves more so in some people over others. 

The US Food and Drug Administration (FDA) has recently highlighted some potential adverse effects that the general public should be aware of, including possible liver injury, medication interactions, male reproductive toxicity, stomach upset, and changes in mood.

When reviewing scientific literature, it’s difficult to find any documented reports of severe side effects from the use of CBD oil. 

Nonetheless, the use of Cannabidiol is still relatively new, and some of the ways to use the compound are new even to those who have previously used cannabis.

Hardcoded Evidence Is Lacking

There is often talk from medical practitioners of how the use of CBD has snowballed so fast that researchers haven’t been able to keep up, and they are correct.

While the compound is a component of cannabis, CBD used in isolation is something new to the most of us, and double-blind clinical studies examining the pros and cons of the cannabinoid are less than abundant.

Still, in both the UK and the USA a version of highly purified pharmaceutical grade CBD is already available in licensed medicine called Epidiolex (Epidoylex in the UK) for use in reducing seizures in severe forms of epilepsy including Lennox-Gastaut syndrome and Dravet syndrome.

Additionally, Sativex is a whole hemp plant US/UK approved medication used to reduce neuropathic pain and the frequency of muscle spasms in multiple sclerosis which contains both CBD and THC in a 1:1 sublingual spray.

These 3 products are medically reviewed products which are of a pharmaceutical standard. As such, they have undergone clinical trials which are useful proxies to give some insight into the potential adverse effects of Cannabidiol. 

Unfortunately, the multiple clinical studies required to licence these medications took decades and a tremendous amount of capital expenditure. For which it’s only in the interest of pharmaceutical companies to carry out if there is an opportunity to both patent a drug and profit from it afterwards.

Nonetheless, while we are currently in a stage of flux in respect to substantial clinical evidence around the benefits of CBD oil and the potential side effects, anecdotal reports will continue to grow as a proxy.

We are also fortunate that an increasing number of clinical trials being funded outside of pharmaceutical companies and the fact that over 125 clinical CBD trials are already active and in progress in the US today

CBD Side Effects In Clinical Research

We still lack a catalogue of clinical evidence for both the use of CBD and the case against CBD oil.

Data from the few high-quality clinical trials which have completed haven’t reported many severe side effects. However, in the US FDA trials for Epidiolex, about 13% of participants had elevated liver enzymes, indicating the need to monitor for liver injury and possibly reducing or stopping dosing CBD altogether if this occurs or in persons with pre-existing liver conditions.

The most significant scientific evidence discussing any CBD side effects have been associated with trials conducted for the treatment of epilepsy with the medicine Epidiolex.

The study consisted of 120 children aged between 2 and 18 treated for 14 weeks, with a total daily dose of between 10mg to 20mg per kg of weight.

For ease of comparison, this would equate to a CBD dosage of between 750mg to 1500mg for an adult weighing 75kg. This is considered an extremely high dose; for comparison, this daily dosage exceeds an entire bottle of the average CBD oil. This would be an impractical dosage for everyday usage.

Common side effects of Epidiolex detailed by the clinical follow-up US Food and Drug Administration report included:

  • Loss of appetite
  • Drowsiness
  • Lethargy
  • Diarrhoea
  • Elevated liver enzymes

While all of these side effects are notable and can be troublesome, most can be termed as mild at best.

However, increased liver enzymes have the most potential for downstream disruption, particularly in those who have a pre-existing liver condition.

side effects of cbd oil

Heightened Liver Enzymes

In the Epidiolex trial three children were removed mid-way from the study due to higher AST and ALT liver enzymes. 

Although, the participants took extremely high doses of CBD and also taking other medications which are known to damage the liver. Some of the liver enzyme elevation reduced by itself while still on Epidiolex, and most elevation had resolved after stopping CBD.

As a result, it’s unclear from this study if CBD had this effect or if it can be indirectly attributed to interactions with other pharmaceuticals. 

There is currently mixed evidence over the protective vs toxic effects of CBD on the liver, however it would likely be best for the average consumer not to take CBD in extremely high doses (such as 20mg/kg) or any more than 70mg CBD per day for health and practical purposes without consulting their doctor for advice.

It’s clear more research on this issue is required before concluding this topic.

Lower Blood Pressure

Outside of the double-blind study mentioned above, there are a variety of other less comprehensive studies which detail the side effects of CBD.

Although, some of these are animal studies, meaning these side effects don’t necessarily line up in humans.

Overarching findings within the current literature include:

Evidence that CBD could impact blood pressure. In a study of 9 healthy men given a 600mg dose of CBD oil vs placebo, the individuals given CBD oil had lowered blood pressure vs the control group. 

The researchers also tested the participants under scenarios which would usually increase blood pressure through stress. In contrast, the blood pressure did increase the magnitude of the impact was smaller than when the participants didn’t take CBD oil.

While reduced blood pressure can be as a positive or negative depending on the individuals’ circumstances, it’s still important to note and discuss.

The CBD Oil Grapefruit Test

medically reviewed cannabidiol

There is also evidence of CBD-induced liver injury in rats treated with ultra-high dose CBD. There is also a concern about reproductive impairment in male mice with very high (up to 30mg/kg) dosages.  

CBD can interact with other drugs which disrupt the way the body processes certain prescription drugs in our body. In particular, CBD has inhibited cytochrome P450 in animal studies, a family of enzymes necessary for the metabolism of common prescription drugs such as the anticoagulant warfarin.

Researchers believe this interaction is particularly effective at blocking the metabolism of medications in the same way grapefruit interacts with several drugs due to CBD deactivating cytochrome P450 enzymes. 

As a result, it is critical individuals who already take prescription drugs talk to their doctor before using CBD oil.

Side Effects From Unfit CBD Oil

Clinical trials use pharmaceutical-grade CBD, which has gone through quality assurances processes to ensure it is fit for use.

On the other hand, the CBD sold online and offline today as a supplement isn’t subject to these same stringent requirements.

Consequently, consumers who purchase CBD oil from brands or manufacturers who haven’t gone through a thorough QA process paired with third party testing, risk the possibility of side effects derived from low-quality CBD oil.

These untested products can contain:

  • Pesticides and Insecticides. If used, they need to be tested for at multiple stages during production and extraction. 
  • Illegal levels of THC. Hemp naturally contains low levels of THC but this concentrates when extracting CBD from the plant. Higher levels of THC can lead to side effects such as dry mouth, hunger and potentially paranoia.
  • Heavy metals. Hemp is a bio-accumulator and can concentrate heavy metals in final products if mitigative steps are not taken.
  • Mould and bacteria. Like any plant, hemp flowers are subject to microbial bacteria in the wrong environment.
  • Zero or otherwise inaccurate CBD content, a UK report outlined a product being sold in a pharmacy containing 0% CBD, reiterating the need for third party batch testing.

As a result, it is crucial if you are going to buy CBD oil that it is tested and the third party certificates of analysis are available online to review.


  • Science examining the benefits and side effects of CBD oil in humans is still limited.
  • Most of the high-quality data around the use of cannabidiol is derived from two clinical trials around the use of CBD for forms of epilepsy. 
  • While considered safe, CBD oil may result in some adverse effects which need to be considered before use.
  • The Cannabidiol industry is mostly unregulated. Purchasing thoroughly tested products can help give you increased information around product quality.
  • Always talk to your doctor before use if you use prescription medication or need medical advice.