Epidiolex Side Effects: What to Expect and When to Call Your Doctor

Epidiolex (cannabidiol) is generally well tolerated, but like all antiseizure medications, it carries side effects that patients and caregivers need to know about. Understanding what's expected — and what's a warning sign — helps you manage your treatment confidently and know when to call your neurologist. Here is a complete breakdown of Epidiolex side effects from clinical trial data and real-world use.

The Most Important Warning: Liver Problems (Boxed Warning)

Epidiolex carries an FDA boxed warning — the most serious type of drug warning — for hepatic injury (liver problems). Epidiolex can cause dose-related elevations of liver enzymes (ALT and AST). In clinical trials for LGS and Dravet syndrome:

13% of Epidiolex-treated patients had ALT elevations greater than 3 times the upper limit of normal, compared to 1% on placebo

Liver enzyme elevations usually occurred within the first 2 months of treatment

The risk is significantly higher when Epidiolex is combined with valproate (valproic acid)

In TSC patients, incidence of ALT >3x ULN was 20-25% in those also taking valproate

Call your doctor right away if you notice: nausea, vomiting, fever, unusual fatigue, loss of appetite, yellowing of the skin or eyes (jaundice), dark urine, or upper right abdominal pain. These may be signs of liver injury.

Your doctor will monitor your liver enzymes with blood tests before starting Epidiolex and then at 1, 3, and 6 months into treatment. Don't skip these tests.

Common Side Effects of Epidiolex (≥10% of Patients)

The following side effects occurred in at least 10% of Epidiolex-treated patients and more often than with placebo in clinical trials:

1. Somnolence and Sedation (32%)

Drowsiness (somnolence) is the most common side effect of Epidiolex. In clinical trials at the 10 and 20 mg/kg/day doses, 32% of patients experienced somnolence or sedation compared to 11% on placebo. This is generally dose-related and often improves over time. The risk is higher in patients also taking clobazam.

What to do: Avoid driving or operating heavy machinery until you know how Epidiolex affects you. If sedation is severe, contact your neurologist — a dose adjustment or a clobazam dose reduction may help. Do not combine Epidiolex with alcohol or other sedating medications without medical guidance.

2. Decreased Appetite

Reduced appetite is common with Epidiolex. This can lead to weight loss over time. Monitor your child's weight regularly, especially in growing children. If appetite loss is significant or the patient is losing weight, speak with your doctor about strategies to maintain nutrition.

3. Diarrhea

Diarrhea is reported in clinical trials. It is often transient and improves as the body adjusts to the medication. If diarrhea is severe or persists, contact your doctor. Staying well hydrated is important.

4. Fever (Pyrexia)

Elevated body temperature (pyrexia/fever) was reported more commonly in Epidiolex-treated patients than placebo in clinical trials. Always contact your doctor if a patient on Epidiolex develops a fever that could also be a sign of infection or liver inflammation.

5. Vomiting

Vomiting can occur, especially early in treatment. Taking Epidiolex with food may reduce nausea and vomiting. If severe or persistent, contact your doctor.

6. Fatigue, Malaise, and Weakness

General tiredness, malaise (feeling unwell), and asthenia (weakness) are common and often occur alongside somnolence. These effects may diminish as your body adjusts. If fatigue is debilitating, discuss dose timing or adjustment with your neurologist.

7. Rash

Rash occurred in Epidiolex clinical trials. If you develop a rash, contact your doctor. While most rashes are mild, any rash with fever, blistering, or widespread skin involvement should be evaluated immediately, as hypersensitivity reactions including angioedema have been reported.

8. Sleep Disturbances

Insomnia and poor-quality sleep have been reported in some Epidiolex patients. This may seem paradoxical given that somnolence is also common — the effect varies between individuals.

Serious Side Effects: When to Call Your Doctor or Seek Emergency Care

Contact your doctor or go to the emergency room immediately if you notice:

Signs of liver injury: jaundice (yellowing skin/eyes), dark urine, right-sided abdominal pain, persistent nausea or vomiting, unusual fatigue

Suicidal thoughts or behavior: Like all antiseizure medications, Epidiolex carries a class warning for increased risk of suicidal thoughts or behavior (affecting approximately 1 in 500 patients). Report any mood changes, depression, anxiety, agitation, or suicidal thoughts immediately.

Severe allergic reactions: Hives, difficulty breathing, swelling of face, lips, tongue, or throat (angioedema) are signs of a serious hypersensitivity reaction — call 911 immediately

Increased seizure frequency: While rare, any significant increase in seizure activity should be reported to your neurologist promptly

Side Effects in the Context of Other Medications

Many side effects are amplified by drug interactions. Patients on clobazam should be closely monitored for excessive sedation, since Epidiolex significantly increases clobazam's active metabolite levels. Patients on valproate need extra vigilance for liver enzyme changes. Always tell your doctor about every medication, supplement, or herbal product you take.

For a detailed look at drug interactions with Epidiolex, see our guide on Epidiolex drug interactions. And if you're having trouble finding your Epidiolex prescription, visit medfinder.com to locate pharmacies near you with it in stock.