CBD Oil Side Effects UK: What Does the Evidence Say?
Can CBD Oil Cause Side Effects?
Yes.
CBD should not be described as completely harmless or “side-effect free”.
The UK Food Standards Agency says CBD consumption can cause effects such as drowsiness, can interact with medicines, and that higher lifetime exposure increases the likelihood of longer-term adverse effects including liver or thyroid problems.
At the same time, it is important not to exaggerate the evidence.
Many of the detailed side-effect figures available for cannabidiol come from trials of prescription medicinal CBD at much higher doses than the amounts covered by current UK consumer-food guidance.
So this guide separates what the FSA currently advises ordinary UK consumers from what has been observed in high-dose medical cannabidiol research. Read the FSA's current CBD safety guidance.
What Is the Current UK FSA Advice on CBD?
As of June 2026, the Food Standards Agency advises healthy adults consuming CBD foods to limit total CBD consumption to 10mg per day.
The FSA says CBD foods consumed in line with this recommendation should not cause acute harm to healthy adults.
That wording is important.
The FSA does not say that every amount of CBD is harmless, and the 10mg figure is not an amount proven to provide a health benefit.
It is consumer-safety guidance, not a therapeutic or effective dose. Read the FSA's 2026 guidance.
Can CBD Make You Drowsy?
Yes.
The FSA specifically says CBD consumption can lead to drowsiness.
It also says that, when healthy adults remain within its provisional acceptable daily intake of 10mg CBD per day, consumption is not expected to make them feel drowsy or sleepy.
That is more accurate than the old version of this article, which described drowsiness as a mild effect associated mainly with taking “too high a dose” and then suggested that this was why people used CBD for sleep.
We should not turn an adverse effect into a health-benefit claim.
Drowsiness is a safety consideration, not evidence that a consumer CBD oil improves sleep. See the FSA's explanation.
What Side Effects Have Been Seen With Prescription Cannabidiol?
The licensed cannabidiol medicine Epidyolex provides useful information because it has been studied in controlled clinical trials.
Reported adverse reactions include somnolence or sedation, decreased appetite, diarrhoea, vomiting, fatigue, nausea, changes in liver enzymes and other effects.
However, those findings require context.
Epidyolex is a prescription medicine used for specific severe epilepsy conditions, under medical supervision, and the recommended doses are generally measured in milligrams per kilogram of body weight per day.
Those amounts can be vastly higher than the FSA's 10mg-per-day guidance for CBD foods.
Therefore, it would be misleading to take an adverse-event percentage from an Epidyolex trial and tell a Valley CBD customer that they have the same percentage chance of experiencing that effect from a consumer CBD oil.
The medicinal research demonstrates that cannabidiol is biologically active and capable of producing adverse effects.
It does not establish product-specific side-effect rates for ordinary retail CBD oils. Read the current Epidyolex Summary of Product Characteristics.
Can CBD Affect the Liver?
This is one of the better-established safety concerns at medicinal doses.
The current Epidyolex prescribing information states that cannabidiol can cause dose-related increases in liver transaminases, which are enzymes commonly measured when assessing liver health.
The risk in medicinal use can also be affected by other medicines, particularly some anti-epileptic drugs such as valproate. See the Epidyolex safety information.
Again, this should not be converted into a claim that ordinary consumer CBD oil routinely causes liver damage.
The dose and clinical circumstances are very different.
But neither should a retailer tell consumers that CBD presents no liver considerations.
The FSA's current consumer guidance specifically says that greater lifetime CBD consumption increases the likelihood of long-term adverse effects such as liver damage or thyroid issues. Read current FSA guidance.
What About CBD and the Thyroid?
The FSA includes thyroid issues among the possible long-term adverse effects associated with greater lifetime exposure to CBD.
That does not mean everyone consuming CBD will develop a thyroid problem.
It means long-term exposure is part of the FSA's safety assessment and is one reason its consumer advice has become substantially more cautious than older CBD guidance. Read the FSA's current position.
This is also why the historic advice repeated on many older UK CBD websites should not be treated as current guidance.
Can CBD Interact With Medication?
Yes.
This is one of the most important safety points in the entire article.
CBD can interact with certain medicines, and the significance of an interaction depends on the particular medication involved.
The FSA's current advice is straightforward:
CBD is not recommended for people taking medication unless under medical direction. Read the FSA guidance.
The previous version of this article relied heavily on the familiar “grapefruit warning” comparison.
Although some drug-metabolism pathways can overlap, using grapefruit warnings as a simple test for whether CBD is safe with a medicine is not reliable enough.
A person taking medication should discuss CBD with an appropriate healthcare professional rather than trying to judge an interaction from a general internet rule.
Who Does the FSA Advise Should Avoid CBD?
Current FSA guidance advises avoidance for vulnerable groups including:
- children under 18
- people who are pregnant
- people who are breastfeeding
- people trying to conceive
- people taking medication
- people with underlying health conditions unless CBD is being used under appropriate medical direction.
This is significantly stronger advice than simply saying “ask your doctor if you are unsure”.
It should therefore appear prominently in a modern UK CBD safety guide. See the FSA's advice.
Are CBD Side Effects Usually Mild and Rare?
We should not make that blanket claim.
The old Valley CBD article repeatedly described side effects as rare, mild and easy to avoid.
There is not enough evidence to attach one universal frequency or severity description to every consumer CBD oil.
Risk can vary with exposure, formulation, individual biology, other medications and underlying health circumstances.
High-dose medicinal trials also demonstrate that cannabidiol can produce clinically important adverse reactions in some circumstances. See the medicinal safety profile.
The evidence-led answer is therefore:
CBD can cause adverse effects, but the likelihood and severity cannot be reduced to one percentage that applies to every CBD product and every person.
Can CBD Cause Diarrhoea or Stomach Problems?
Gastrointestinal adverse effects have been observed in clinical cannabidiol research.
For Epidyolex, diarrhoea and vomiting are among the commonly reported adverse reactions, while nausea is also reported. Read the current prescribing information.
But once again, those are findings from medicinal use at much greater CBD exposures.
They should not be presented as side-effect frequencies for Valley CBD oil.
It is also possible for ingredients other than CBD to influence how somebody tolerates a finished product.
So it is too simplistic to assume that every digestive symptom after taking a CBD oil must have been caused by either the CBD or its carrier oil.
Can CBD Affect Appetite?
Decreased appetite is among the adverse reactions reported with prescription cannabidiol. See the Epidyolex safety information.
That does not prove that ordinary CBD food supplements routinely change appetite, and we would not advertise appetite alteration as either an expected effect or a potential benefit.
Does CBD Cause Dry Mouth?
“Dry mouth” or “cottonmouth” appears frequently on CBD websites.
However, it is not one of the central consumer risks highlighted in the FSA's current CBD guidance, nor is it among the principal adverse reactions emphasised in the current Epidyolex safety profile.
We therefore should not present it as though its frequency in ordinary CBD-oil users is firmly established.
This is another example of why repeating generic lists from old CBD articles can create more certainty than the evidence supports.
Is CBD Safe for Long-Term Use?
There is still uncertainty around long-term consumer exposure.
The FSA's current position is based partly on concern about cumulative lifetime exposure.
Its guidance says that the more CBD someone consumes over their lifetime, the greater the likelihood of long-term adverse effects such as liver damage or thyroid issues. Read the FSA guidance.
That means we should not describe long-term CBD use as proven safe.
Nor should we tell consumers that gradually increasing the amount until they find their “sweet spot” is inherently safe.
The previous version of this page did exactly that.
We are removing that advice.
Does CBD Isolate Mean There Are No Side Effects?
No.
CBD isolate contains CBD without the wider range of cannabinoids found in some other cannabis extracts.
But cannabidiol itself is still the active compound being discussed in the FSA safety guidance and medicinal research.
Using isolate therefore does not make the general safety considerations around CBD disappear.
That distinction is important for Valley CBD because our oils use CBD isolate rather than broad-spectrum CBD.
Product composition should be described accurately, but it should not be used to imply that CBD itself cannot produce adverse effects.
Is CBD Addictive?
This is a slightly different question from side effects and deserves its own evidence review.
Rather than duplicating that topic here, our separate Is CBD Addictive? What Does the Research Actually Show? article examines dependence and abuse potential in detail.
For this page, the important distinction is simply that addiction risk and general safety are not the same question.
A substance does not have to be addictive to have side effects, interactions or safety limits.
What Should You Do if You Think CBD Is Causing a Side Effect?
If someone becomes unwell after consuming CBD, experiences unexpected effects or is concerned about a possible medicine interaction, they should stop relying on general online advice and speak with an appropriate healthcare professional.
Severe or concerning symptoms require appropriate medical attention.
A CBD retailer should not attempt to diagnose the cause of symptoms or advise somebody to adjust medication.
Does Valley CBD Have a Product-Specific Side-Effect Rate?
No.
Valley CBD's finished retail oils have not undergone the type of large controlled clinical safety trial that would allow us to publish a reliable product-specific percentage for drowsiness, gastrointestinal effects or other adverse events.
We should therefore keep two things separate:
Valley CBD product facts tell customers what the product contains and its CBD concentration.
Independent scientific and regulatory evidence tells readers what is currently known about cannabidiol safety more generally.
That is a more transparent approach than claiming our particular oil is “side-effect free” or “safe for everyone”.
Frequently Asked Questions
What are the main side effects of CBD?
The FSA specifically highlights drowsiness, medicine interactions and longer-term concerns including liver and thyroid effects with greater lifetime exposure.
High-dose prescription cannabidiol trials have also reported effects including diarrhoea, decreased appetite, vomiting, fatigue, nausea and changes in liver enzymes, although those medicinal-dose findings should not be directly applied to ordinary consumer CBD products. Read current FSA guidance.
Can CBD oil make you sleepy?
Yes, CBD can cause drowsiness.
The FSA says CBD consumed within its 10mg-per-day guidance is not expected to make a healthy adult drowsy or sleepy, although individual circumstances can differ. Read the FSA guidance.
Is 10mg CBD per day an effective dose?
No.
The FSA's 10mg figure is consumer-safety guidance.
It is not a treatment dose and does not establish an amount that produces a health benefit.
Can CBD interact with prescription medicines?
Yes.
The FSA advises people taking medication to avoid CBD unless under medical direction. Read current FSA guidance.
Can CBD damage the liver?
High-dose medicinal cannabidiol can cause dose-related liver-enzyme elevations, and current FSA advice identifies liver damage as a potential long-term concern with greater lifetime CBD exposure.
That does not establish a specific liver-risk percentage for ordinary retail CBD oils. Read the medicinal safety information.
Is CBD safe during pregnancy?
The FSA advises people who are pregnant, breastfeeding or trying to conceive to avoid CBD. Read the FSA advice.
Are CBD side effects always mild?
No universal claim like that can be made.
Effects and risks depend on exposure and individual circumstances, and clinically important adverse reactions have been documented with high-dose medicinal cannabidiol.
Does CBD isolate avoid CBD side effects?
No.
CBD isolate removes many other cannabis compounds, but it still contains cannabidiol. General safety advice concerning CBD therefore remains relevant.
The Bottom Line
CBD should neither be presented as dangerous by default nor advertised as completely harmless.
For UK consumers, the clearest starting point is current FSA guidance.
Healthy adults are advised to consume no more than 10mg CBD per day from CBD foods, while children, people who are pregnant or breastfeeding, people trying to conceive and people taking medication are advised to avoid CBD. People with underlying health conditions are also advised to avoid it unless under medical direction. Read the FSA guidance.
CBD can cause drowsiness and can interact with medicines.
Greater lifetime exposure is also associated in the FSA's risk assessment with an increased likelihood of long-term adverse effects such as liver damage or thyroid issues.
Prescription cannabidiol research confirms that CBD is biologically active and can cause adverse reactions, but those trials generally involve substantially greater medicinal exposures and should not be used to invent side-effect percentages for consumer CBD oils. Read the Epidyolex Summary of Product Characteristics.
The responsible conclusion is simple:
CBD has genuine safety considerations, and those should be explained accurately without exaggerating them or dismissing them.
This article is for general educational information only and is not medical advice. Valley CBD products are not intended to diagnose, treat, cure or prevent disease.