Efavirenz: when a standard dose is more than the body can clear
Efavirenz has a reputation for strange dreams and daytime fog, and for a large group of people that reputation comes down to a dosing problem. They are getting more drug from a standard tablet than the label ever intended, and one gene says who they are.
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What efavirenz is
Efavirenz is an antiretroviral drug used to treat HIV, taken once a day as part of a combination that keeps the virus suppressed. It has been one of the most widely used HIV drugs worldwide, often in fixed-dose combination tablets, though newer options have taken over first-line use in many countries. Its most talked-about side effects are on the nervous system: vivid dreams, dizziness, trouble concentrating and sleep disturbance, usually worst in the first weeks.
Why your genes come into it
CYP2B6 is the main enzyme that clears efavirenz, and reduced-function versions of it are common, particularly in people of African ancestry. Slow metabolizers clear the drug less efficiently, so a standard 600 mg dose leaves them with a higher blood level than it does most people, and that raised level tracks with the nervous-system side effects the drug is known for. It is a clean dose-exposure story: the gene does not change what efavirenz does, only how much of it a standard dose delivers. CPIC responds by describing lower daily doses for intermediate and poor metabolizers, which studies have found can keep the virus suppressed while easing the side effects.
What published guidance says
The rows below restate CPIC's published guidance for each possible CYP2B6 result. They describe what an HIV physician weighs when setting a dose, not a change for anyone to make alone. HIV treatment works by keeping the virus fully suppressed, and altering an antiretroviral dose without the prescriber managing it risks that suppression, which is the one thing the treatment cannot afford to lose.
CYP2B6
| If your CYP2B6 result is | What that means | Published CPIC guidance |
|---|---|---|
| Intermediate Metabolizer | Higher dose-adjusted trough concentrations of efavirenz compared with normal metabolizers; increased risk of CNS adverse events | Per CPIC (A): "Consider initiating efavirenz with decreased dose of 400 mg/day" |
| Normal Metabolizer | Normal efavirenz metabolism | Per CPIC (A): "Initiate efavirenz with standard dosing (600 mg/day)" |
| Poor Metabolizer | Higher dose-adjusted trough concentrations of efavirenz compared with normal metabolizers; significantly increased risk of CNS adverse events and treatment discontinuation | Per CPIC (A): "Consider initiating efavirenz with decreased dose of 400 or 200 mg/day" |
| Rapid Metabolizer | Slightly lower dose-adjusted trough concentrations of efavirenz compared with normal metabolizers | Per CPIC (A): "Initiate efavirenz with standard dosing (600 mg/day)" |
| Ultrarapid Metabolizer | Slightly lower dose-adjusted trough concentrations of efavirenz compared with normal metabolizers | Per CPIC (A): "Initiate efavirenz with standard dosing (600 mg/day)" |
Can your DNA file answer this?
CYP2B6 is not part of the standard chip or variant-file report. Its star alleles are built from combinations of variants spread across the gene rather than from single marker positions, so resolving a diplotype needs sequence. Aimosti reads it from aligned reads in a BAM or CRAM file as part of the Deep Read panel, and where the reads do not settle the genotype the result is reported as Indeterminate rather than an assumed normal. A consumer chip file cannot answer this one.
A genotyping chip reads only a few hundred thousand pre-selected positions, about 0.02% of your genome, chosen for common variation. It does not sequence the rest, so it cannot find the rare or novel pathogenic variants the clinical and carrier modules look for: a “no finding” from chip data means “this chip never looked”, far more so than with whole-genome sequencing. Chip data suits common-variant traits, pharmacogenomic tag SNPs, and haplogroup ancestry, not clinical or carrier screening.
Common questions
Efavirenz gives me strange dreams. Is that my genes?
It might be part of it. Slow CYP2B6 metabolizers run higher efavirenz levels on a standard dose, and higher levels are linked to exactly these nervous-system effects. But these side effects also happen in normal metabolizers, especially early on, and often settle over the first weeks, so a genotype is one piece of the explanation rather than the whole of it. It is worth raising with the prescriber either way.
Could a lower dose fix the side effects?
For some people, yes, which is why the guidance describes reduced daily doses for intermediate and poor metabolizers. The catch is that any dose change has to keep the virus suppressed, so it is a decision for the HIV team with your viral load in front of them, not something to try independently.
Why is reduced CYP2B6 function more common in some populations?
Because the reduced-function alleles vary in frequency by ancestry, and the main one is considerably more common in people of African descent. That is part of why efavirenz dosing became a notable pharmacogenomic example: a standard global dose sat too high for a large share of the people taking it.