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Side Effects of Finasteride (Proscar, Propecia)


Description: Finasteride, often referred to by its trade names Proscar and Propecia, is a medication of the “5-alpha-reductase inhibitor” pharmacological class. It also known as an antiandrogen. It is commonly used to treat a number of conditions including benign prostatic hyperplasia (BPH) and male pattern baldness (MPB). Finasteride works by inhibiting type II and III 5-alpha-reductase. This enzyme converts testosterone to dihydrotestosterone, a molecule that induces growth of the prostate. Reduction in blood concentration of DHT reduces prostate volume, alleviating some symptoms associated with BPH.
In male pattern baldness reduction in DHT production reduces steroid hormone activity.

Serious Propecia Side Effects

According to research reported in Future Oncology, “Finasteride metabolism and pharmacogenetics: new approaches to personalized prevention of prostate cancer,” (Dec 2010), finasteride has been shown to cause prostate cancer in patients who are genetically predisposed, as well as male breast cancer.

Common Propecia Side Effects

As finasteride inhibits the production of androgens, which have myriad effects throughout the body, its side effects are widespread. According to the Propecia drug label as reported by the FDA, some common side effects:

  • Impotence
  • Abnormal ejaculation
  • Abnormal sexual function
  • Erectile dysfunction
  • Gynecomastia (enlargement of male breast tissue)
  • Testicular pain
  • Male infertility
  • Poor sperm quality
  • Decreased ejaculatory volume
  • Decreased libido
  • Dizziness
  • Somnolence
  • Less Common Propecia Side Effects

    Less common side effects include:

  • Postural hypotension
  • Hypotension
  • Edema
  • Understand Your Risks with the Rxight® Genetic Test

    Variable metabolic reaction to finasteride may result in an increased probability of developing adverse effects to the drug. Identifying individual variants in genes (called “polymorphisms”) that metabolize the drug can help a physician get the correct dose for a patient. This can potentially reduce the chances of a patient developing side effects when taking Finasteride.


    MD Labs’ pharmacogenetic testing program, Rxight® helps identify common polymorphisms in enzymes that metabolize Finasteride, along with hundreds of other clinically significant over-the-counter and prescription medications. Knowing how your body is likely to respond to a medication based on your unique genetics will enable you and your prescriber(s) to determine the safest, most efficacious dose, or find a safer alternative if needed – preferably before treatment begins.

    How Does Pharmacogenetic Testing Work?

    Many patients never develop adverse effects when taking finasteride, while others suffer severe adverse reactions. This inter-patient variability has been suggested to be the result of variable metabolism of finasteride, according to Future Oncology (Dec 2010), which reported that polymorphisms in the enzymes that metabolize the drug may result in variable bioavailability of the drug. Finasteride is metabolized by the cytochrome P450 superfamily of enzymes, primarily the isoenzymes CYP3A4. A number of alleles in this enzyme have been associated with variable dose response effects of the drug.


    Identifying which polymorphisms an individual holds can aid physicians in prescribing the correct dose of a drug like Finasteride. This may reduce the probability of an individual developing adverse side effects when taking the drug. Rxight® specifically identifies polymorphisms in many genes (including CYP3A4) to ascertain how individuals are likely to react – specifically whether they will derive no therapeutic benefit because they metabolize the drug more quickly than normal, or whether they will be at risk for side effects because they metabolize the medication slowly.


    Contributors to this Article:
    Michael Sapko, MD, PhD; Deborah Kallick, PhD, Medicinal Chemistry


    Read more about Rxight® pharmacogenetics