CYP2C19 rs41291556: Understanding the *8 Variant
The rs41291556 variant is a specific genetic change within the CYP2C19 gene, commonly referred to as the CYP2C19*8 allele. It is associated with reduced enzymatic activity, which may influence how an individual's body processes certain medications.
What each genotype means
Normal enzyme activity
This genotype represents the wild-type or normal function state for this specific variant. You do not carry the CYP2C19*8 loss-of-function allele at this position. Please discuss any medication-related concerns with your clinician or pharmacist.
This is the most common genotype, found in the vast majority of the global population.
Reduced enzyme activity
You carry one copy of the CYP2C19*8 loss-of-function allele, which is associated with decreased enzymatic activity. This may impact how your body metabolizes certain medications processed by the CYP2C19 enzyme. Please discuss your medication options and potential dosing adjustments with your clinician or pharmacist.
This genotype is uncommon, occurring in approximately 0.1-0.5% of European and admixed American populations.
Decreased enzyme activity
You are homozygous for the CYP2C19*8 loss-of-function allele, which is associated with significantly reduced or absent enzymatic activity. This may impair the metabolism of specific drugs, potentially affecting their clinical effectiveness. Please consult with your clinician or pharmacist to review your medication profile and discuss appropriate management.
This genotype is very rare, found in a small fraction of individuals within European and admixed American populations.
What is rs41291556?
The rs41291556 variant is a single nucleotide polymorphism (SNP) located on chromosome 10 within the CYP2C19 gene. In genetic nomenclature, this specific change is identified as the CYP2C19*8 allele. At the molecular level, this variant involves a substitution where a thymine (T) is replaced by a cytosine (C) at a specific position in the gene sequence. This change results in a missense mutation, specifically p.Trp120Arg, which alters the amino acid sequence of the resulting protein. Because this alteration occurs in a critical region of the enzyme, it is classified as a loss-of-function variant. This means the enzyme produced by this version of the gene may not function as efficiently as the standard, or 'wild-type,' version of the enzyme.
The Role of the CYP2C19 Gene
The CYP2C19 gene provides the instructions for creating an enzyme that belongs to the cytochrome P450 family. These enzymes are primarily located in the liver and are essential for the metabolism of a wide variety of substances, including many commonly prescribed medications. By breaking down or activating these drugs, the CYP2C19 enzyme helps regulate their concentration in the bloodstream. When a person carries a variant like rs41291556, the efficiency of this metabolic process can be impaired. If the enzyme is less active, certain drugs may not be activated properly, or they may remain in the system for longer than intended. Understanding these variations is a core component of pharmacogenomics, a field that studies how an individual's genetic makeup influences their response to drug therapy.
Research and Clinical Evidence
Research into rs41291556 has primarily focused on its impact on enzymatic activity in laboratory settings. Studies have indicated that this variant is associated with a significant reduction in the metabolism of specific substrates, such as S-mephenytoin and tolbutamide. While the evidence clearly points to a loss of function, the clinical implications for specific drug therapies are considered limited compared to more common variants like *2 or *3. Because this variant is relatively rare, large-scale clinical trials specifically targeting the *8 allele are less common. Consequently, while the biochemical effect is well-documented in vitro, its overall impact on patient outcomes for specific medications remains an area of ongoing study. It is important to note that genetic testing results should always be interpreted by a healthcare professional who can consider the full clinical context.
Navigating Your Genetic Information
If you have received information about your rs41291556 genotype, it is natural to have questions about what it means for your health. It is crucial to understand that this information is only one piece of a much larger puzzle. Many factors, including age, liver function, diet, and other medications you may be taking, also influence how your body processes drugs. You should never use genetic test results to make decisions about starting, stopping, or changing the dosage of any medication. If you are concerned about how your body might respond to a specific treatment, the best course of action is to discuss your genetic profile with your doctor or a clinical pharmacist. They can provide personalized guidance based on established clinical guidelines and your unique medical history.
How common is this variant?
The rs41291556 variant is considered an uncommon genetic change, typically found in European and admixed American populations at a frequency of approximately 0.1% to 0.5%.
Frequently asked questions
What does it mean to be a 'slow metabolizer'?
A slow metabolizer has genetic variants that cause their liver enzymes to work less efficiently than normal. This can lead to higher levels of certain drugs in the blood, potentially increasing the risk of side effects or changing how well a medication works.
Does having the *8 variant mean my medication won't work?
Not necessarily. While the *8 variant is associated with reduced enzyme activity, many other factors influence drug response. You should consult your healthcare provider to determine if your specific medication is affected by this variant.
Should I get tested for CYP2C19 variants?
Genetic testing for pharmacogenomics is typically recommended by a doctor when there is a specific clinical need, such as when a patient is not responding to a standard treatment. You should discuss the necessity of such testing with your physician.
Can I change my CYP2C19 genotype?
No, your genotype is determined at birth and cannot be changed. However, knowing your genotype can help your doctor make more informed decisions about your medical care.
Sources & further reading
Educational information only, last refreshed 9/28/2026. Not medical advice — these associations describe population statistics, not individual predictions.
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Variant in the CYP2C19 locus associated with metabolism of clopidogrel and proton-pump inhibitors.
This variant is linked to the loss of function in the CYP2C19 enzyme, affecting clopidogrel efficacy.
Non-functional allele that causes poor metabolism of clopidogrel and PPIs.
Defines the CYP2C19*4 allele caused by an ATG start codon alteration that abolishes translation, resulting in reduced or absent drug metabolism capacity.
Defines the non-functional CYP2C19*3 null allele resulting from a premature stop codon, leading to poor metabolism of drugs such as clopidogrel, escitalopram, and proton pump inhibitors.
The CYP2C19*2 loss-of-function splicing defect results in impaired biotransformation of prodrugs including clopidogrel and altered clearance of proton pump inhibitors and select antidepressants.