Can a Blood Test for Finasteride Determine the Risk of Side Effects?

Based on clinical data (so far), there aren’t yet reliable blood tests to determine someone’s risk of sexual side effects from drugs like finasteride or dutasteride. Having said that, blood tests can likely be used to determine someone’s risk of gynecomastia from the drugs.

Gynecomastia and Hair Loss Drugs

Gynecomastia is the growth of male breast tissue. It results from elevated hormones such as prolactin and estrogen.

While gynecomastia is estimated to only affect between 0.25-1.00% of people on 5-alpha reductase inhibitors. Those who start the drug while already having elevated levels of prolactin and estrogen are likely at a higher risk of its development.


Because drugs like finasteride and dutasteride can raise blood levels of both testosterone and estrogen by 10-30%, depending on the dose.

So, for those starting the drug with borderline-high estrogen, the additional lift in estrogen levels may put someone in the “danger zone” for gynecomastia.

Blood Tests for Finasteride and Dutasteride

For peace of mind, people can always order blood tests for prolactin and estrogen prior to starting finasteride or dutasteride. Additional tests can be performed further down the line.

If levels are within range, the risk of gynecomastia is likely much lower. This can be done with a primary care physician. Those based in the U.S. (and other countries that offer direct-to-consumer lab testing), can order tests through the links below.

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What a Blood Test for Finasteride Can Do

While tests cannot determine the risk of sexual side effects, the genetic test for finasteride response does provide some insight into the potential success rate. Referred to as the “CAG repeat score,” the test can fairly accurately determine the response rate to the drug.[3]https://pubmed.ncbi.nlm.nih.gov/31949455/

Those with certain genetic sequencing may not respond favorably to finasteride. Learning this information can point those people to alternative treatments, thus not only avoiding the risk of side effects but avoiding time and money spent on ineffective hair regrowth solutions.

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