CBD Side Effects and Genetics: What the Evidence Shows

You tried CBD oil for anxiety, or sleep, or chronic pain. Instead of the calm that everyone on the internet promised, you got diarrhea. Or brain fog. Or you slept for twelve hours and woke up groggy. Your friend takes the same product — same brand, same dose — and feels genuinely relaxed with no side effects.

Genetics is a plausible explanation for that difference. It is also, at present, a poorly evidenced one. This article sets out what has actually been measured in people, what the single relevant clinical study did and did not find, and where the honest answer is that nobody knows yet.

These Side Effects Are Real and Well Documented

Whatever the cause, the side effects themselves are not imaginary. In the trials supporting FDA approval of Epidiolex — a prescription CBD oral solution — the adverse reactions occurring in at least 10% of treated patients and more often than with placebo were somnolence, decreased appetite, diarrhea, transaminase (liver enzyme) elevations, fatigue and malaise, rash, sleep problems, and infections.

Two things follow. Sleepiness and diarrhea are among the most common effects of CBD, not unusual reactions. And those figures come from patients on prescription-strength doses, far higher than a typical over-the-counter product — a distinction that matters for everything below.

How Your Body Processes CBD

The FDA label for Epidiolex states that cannabidiol is metabolized in the liver and the gut, primarily in the liver, by CYP2C19 and CYP3A4. Laboratory work on human liver microsomes agrees: CYP3A4 and CYP2C19 are the major cytochrome P450 enzymes acting on CBD, and the conversion of CBD to 7-OH-CBD specifically is predominantly catalyzed by CYP2C19.

7-OH-CBD is the primary active metabolite, and it is roughly equipotent with CBD itself. Later work found that CYP2C9 also contributes to forming it, and that CYP3A accounts for much of CBD's clearance through oxidation at other positions on the molecule. CBD is additionally processed by phase 2 (glucuronidation) enzymes.

One point is worth stating plainly, because it is widely repeated online and appeared in an earlier version of this article: CYP3A5 is not an established primary metabolizing enzyme for CBD. The enzymes named in the FDA label and in the reaction-phenotyping literature are CYP2C19 and CYP3A4.

The One Study That Looked at Genotype and CBD Side Effects

In January 2026, a team at Indiana University School of Medicine published what appears to be the only clinical study to date examining CBD adverse events by genotype. It is a secondary analysis of an open-label study in 33 healthy volunteers, who received a single oral dose of CBD at 5 mg/kg and were later titrated to 5 mg/kg twice daily for 14 days.

The headline finding:

Diarrhea occurred in 7 of 18 participants (39%) who were both CYP3A5 non-expressers (termed CYP3A5 poor metabolizers in the paper) and CYP2C19 intermediate or normal metabolizers, compared with 1 of 15 (7%) among everyone else, p = 0.0463. That same group also had higher steady-state exposure to the metabolites 7-OH-CBD and 7-COOH-CBD.

That is a real, peer-reviewed result. It is also a far weaker basis for a genetic test than it first appears — and the reasons are all stated by the authors themselves.

Why That Result Does Not Support a Genetic Test

• The comparison rests on a single event. The 7% figure is one person out of fifteen. Any ratio built on one event moves enormously if that one person had been counted differently.
• The two genes could not be separated. CYP2C19 intermediate and normal metabolizers accounted for 7 of the 8 diarrhea cases in the study. The authors state that these participants could not be analyzed separately because of substantial overlap with the CYP3A5 non-expresser group. This is therefore not established as a two-gene interaction — the signal is largely a CYP2C19 signal that CYP3A5 could not be disentangled from.
• The authors do not claim cause and effect. They write explicitly that they do not suggest a cause-and-effect relationship between plasma 7-OH-CBD or 7-COOH-CBD concentrations and diarrhea.
• The pharmacokinetic effects were modest. The paper describes the genotype-associated changes as mild drug–gene interactions, with most changes in drug exposure comparable in magnitude to a mild drug–drug interaction.
• It was a small secondary analysis. The authors list this as a limitation, along with having examined only phase 1 metabolism, and note that grouping metabolizer phenotypes the way they did raises the risk of statistical error.
• These were not wellness doses. Participants took 5 mg/kg twice daily — for a 70 kg adult, roughly 350 mg twice a day. Typical over-the-counter CBD products deliver a small fraction of that. No study has tested whether genotype affects side effects at those doses.

The authors' own conclusion is the clearest statement of all: given the wide therapeutic window and range of CBD doses, they write, pharmacogenomic testing may hold questionable utility for predicting adverse events related to plasma concentrations.

The paper also carries a published correction, issued in February 2026, which transposed two of its figures. It does not affect the diarrhea results described here.

Genotype Is an Imperfect Proxy for Enzyme Activity

There is a further, more fundamental limit. In genotyped human liver microsomes, formation of 7-OH-CBD correlated with CYP2C19 activity but was not associated with CYP2C19 genotype. Enzyme activity is shaped by other medications, liver health, and factors a DNA test cannot see.

The clinical study hit the same wall from the other direction. It found CBD blood levels went up as CYP2C19 activity increased — the opposite of the straightforward expectation. The authors call this paradoxical, note that the same unexpected direction appears in the FDA Epidiolex clinical pharmacology review, and state that the underlying mechanism is unknown.

Taken together: the evidence does not currently support using a CYP3A5 or CYP2C19 result to predict who will get diarrhea from CBD.

What Is Well Documented: CBD as an Enzyme Inhibitor

The clinically meaningful risk with CBD is not really about how fast you clear it. It is about what CBD does to the enzymes that clear other drugs. The FDA label states that cannabidiol has the potential to inhibit CYP2C8, CYP2C9, and CYP2C19 at clinically relevant concentrations, and advises considering a dose adjustment when Epidiolex is combined with a moderate or strong inhibitor, or a strong inducer, of CYP3A4 or CYP2C19.

The best-documented example is clobazam. In 13 children with refractory epilepsy taking both drugs, adding CBD raised clobazam levels by a mean of 60% and levels of its active metabolite norclobazam by a mean of 500%. Side effects appeared in 10 of the 13 and were relieved by reducing the clobazam dose.

If you take prescription medication and are considering CBD, this is the part that matters. Our guide to CBD drug interactions covers which drug classes are affected.

Other Genes That Come Up With Cannabis

For THC rather than CBD, the evidence for a genetic effect is somewhat stronger. In 43 healthy volunteers given oral THC, people carrying two copies of the reduced-function CYP2C9*3 variant had roughly threefold higher total THC exposure than people with two normal-function copies, along with a trend toward greater sedation. We cover that in why edibles affect people differently, and take a broader look at cannabis-related genes in our guide to cannabis genetics from 23andMe data.

If CBD Gives You Side Effects

The practical picture is simpler than the genetic story. CBD side effects are common and dose-related, and the reliable levers are the dose, the product, and your other medications — not your genotype. Because CBD interacts with prescription drugs through enzyme inhibition, the single most useful thing you can do is tell your doctor and pharmacist that you take it, exactly as you would report any other drug.