Our bodies break down (metabolize) drugs and other toxins through a group of enzymes known as the CYP450 family. Different CYP enzymes break down different substances, and we all carry genetic variants that can impact whether we metabolize a drug quickly or slowly.

The CYP3A genes (which code for enzymes of the same name) is a subfamily of CYP 450 and is involved in the metabolism of about half the drugs on the market today as well as other xenobiotics and steroids.  There are several major genetic polymorphisms in the CYP3A family that can play a role in how a person reacts to a medication.

Several fruits – grapefruit, noni, pomegranate – are potent inhibitors of CYP3A4.  Eating or drinking these can cause adverse effects on drug metabolism, either increasing the efficiency of the drug or decreasing the effect.

Interestingly, CYP3A4 is naturally more active in women than in men.

A long list of drugs metabolized by CYP3A4 is available on Pharmacy Times and on  Wikipedia.  Some very commonly used drugs on the list include Viagra, Xanax, Cialis, and Zocor.  Methadone dosages can also be impacted by this gene.[ref]

CYP3A4 Gene Variants:

Check your 23andMe results for rs4987161 (v4, v5):

  • GG: CYP3A4*17, decreased function of enzyme, [ref] [ref]
  • AG: carrier of one CYP3A4*17 allele
  • AA: normal

Check your 23andMe results for rs4986909 (v4 only):

  • AA: CYP3A4*13, decreased function of the enzyme, [ref]
  • AG: carrier of one CYP3A4*13 allele
  • GG: normal

Check your 23andMe results for rs2740574 (v4,v5):

  • CC: CYP3A4*1B, decreased function of the enzyme [ref]
  • CT: carrier of one CYP3A4*1B allele
  • TT: normal

Check your 23andMe results for rs4986910 (v4, v5):

  • GG: CYP3A4*3, decreased function
  • AG: carrier of one CYP3A4*3 allele
  • AA: normal

Check your 23andMe results for rs4986907 (v4, v5):

  • TT: CYP3A4*15A, decreased function
  • CT: carrier of one CYP3A4*15A allele
  • CC: normal


CYP3A5 Gene Variants:

CYP3A5 is another enzyme that is found in the liver, prostate, and intestines. The vast majority of Caucasians have a genetic variant (rs776746) that decreases the functionality of this enzyme.

Please note: Most of the drugs metabolized through CYP3A4 are also metabolized by CYP3A5. So if you are a poor or non-functioning CYP3A5 metabolizer but have functioning CYP3A4 genes, you may be just fine.  Since the majority of several populations have the non-functioning variants, for this gene NOT carrying a variant may impact your metabolism of certain drugs by increasing enzyme function.[ref]

In addition to several types of drugs, CYP3A5 also metabolizes testosterone, progesterone, and androstenedione.   [ref]

Check your 23andMe results for rs776746 (v4, v5):

  • CC:  CYP3A5*3, non-functional –most common type in Caucasians–
  • CT: carrier of only one CYP3A5*3 allele (thus some CYP3A5 function)
  • TT: normal function


Check your 23andMe results for rs41303343 (v4):

  • AA:  CYP3A5*7, non-functional
  • -A: carrier of one CYP3A5*7 allele (thus some CYP3A5 function)
  • II: normal function


Check your 23andMe results for rs28365083 (v4, v5):

  • TT:  CYP3A5*2, non-functional
  • GT: carrier of one CYP3A5*2 allele (thus some CYP3A5 function)
  • GG: normal function


Check your 23andMe results for rs55817950 (v4, v5):

  • AA:  CYP3A5*8, non-functional
  • AG: carrier of one CYP3A4*8 allele (thus some CYP3A5 function)
  • GG: normal function


Check your 23andMe results for rs28383479 (v4, v5):

  • TT:  CYP3A5*9, non-functional
  • CT: carrier of one CYP3A4*9 allele
  • CC: normal


Check your 23andMe results for rs41279854 (v4, v5):

  • GG:  CYP3A5*10, non-functional
  • AG: carrier of one CYP3A4*10 allele (thus some CYP3A5 function)
  • AA: normal function


Check your 23andMe results for rs56244447 (v4, v5):

  • CC:  CYP3A5*3D, non-functional
  • AC: carrier of one CYP3A4*3D allele (thus some CYP3A5 function)
  • AA: normal function


Grapefruit, noni, and pomegranate juices are inhibitors of CYP3A4 and CYP3A5. If you carry any of the genetic variants above for CYP3A4, be very careful of these juices or other inhibitors when taking a medication that is metabolized through CYP3A4.

St. John’s Wort is an inducer of CYP3A4, meaning it causes more of the enzyme to be created in the body. If you are taking a drug that is metabolized by CYP3A4, then you need to be careful with St. John’s wort interacting with the medication. (Talk with a doctor or pharmacists)[ref]

This is part of an ongoing series on the genes involved in detoxification.

1 Comment

genelife · July 18, 2017 at 2:01 pm

Hi Leon – Sorry to take so long to respond to your comment on my blog. I do appreciate you reading the blog, and I really appreciate you letting me know that I had the description on those alleles flipped around!

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