Finasteride has a relatively short list of contraindications and almost does not interact with other drugs, but some of its limitations apply not only to the patient, but also to the people around him. The editors considered what is important to know before starting treatment.
Feature of finasteride: the main risk is for a third party
Most of the contraindications concern the person taking the drug. In finasteride, the most important limitation is related to another person - a male fetus. Dihydrotestosterone (DHT) is necessary for the normal formation of a boy's external genitalia, and its deficiency in the fetal period can lead to developmental abnormalities.
This risk is known from a natural experiment: in men with a hereditary deficiency of 5α-reductase, sexual development disorders are observed. Pharmacological inhibition of the same enzyme during pregnancy can theoretically reproduce similar changes.
That is why the instructions for the drug pay so much attention to women who do not even take finasteride, but can only come into contact with it.
Below, the editorial systematizes contraindications, precautions, and known interactions based on official Proscar and Propecia instructions and clinical guidelines.
Absolute contraindications
Official information highlights the following situations in which the drug is not used:
- pregnancy or the possibility of pregnancy (the drug is contraindicated for women of reproductive age);
- child age;
- hypersensitivity to finasteride or excipients;
- for some forms — lactose intolerance (depending on the composition of the tablet).
Women who are or may be pregnant should not handle crushed or broken tablets, as the active substance may be absorbed through the skin. Whole tablets are covered with a shell that prevents contact with the active substance during normal handling.
Regarding semen: the instructions state that the amount of finasteride that can enter the partner with semen is very small and is not considered a risk to the male fetus. However, the issue of pregnancy planning should be discussed with a doctor.
The drug is also not prescribed to children and adolescents: effectiveness and safety in this age group have not been established.
Conditions requiring caution
Finasteride is actively metabolized in the liver. Data on patients with hepatic impairment are limited, so the guidelines advise caution in such cases.
Before starting treatment for prostatic hyperplasia, the doctor must rule out prostate cancer and other diseases that can mimic BPH. Patients with significant residual urine volume or reduced flow require careful monitoring due to the risk of obstructive uropathy.
The drug reduces PSA by approximately half. The physician should be aware of finasteride intake when interpreting screening results. According to the PCPT (Thompson et al., 2003) and the corresponding 2011 FDA warning, high-grade tumors were more common among men on finasteride, so any sustained increase in PSA should be investigated.
A history of depression is a reason for special attention: the instructions contain warnings about mood changes and suicidal thoughts. The patient and relatives should be informed about the need to seek help when such symptoms appear.
Drug interactions
Finasteride is metabolized mainly with the participation of the CYP3A4 enzyme. Despite this, no clinically significant interactions were found in the studies. The instructions note the absence of important interactions with propranolol, digoxin, glibenclamide, warfarin, theophylline and antipyrine.
Strong inhibitors or inducers of CYP3A4 could theoretically alter the concentration of finasteride, but given the wide therapeutic window of the drug, this is rarely of practical importance.
Combination with alpha-blockers (doxazosin, tamsulosin) is standard practice in prostatic hyperplasia, confirmed by the MTOPS study. The combination reduces the risk of progression more effectively than each drug alone.
Simultaneous use of two 5α-reductase inhibitors (finasteride and dutasteride) does not make sense. Herbal remedies with similar effects, such as palm serena extract, should be discussed with your doctor to avoid duplication and false expectations.
| Drug / situation | Interaction | Practical conclusion |
|---|---|---|
| Alpha blockers | Complement the action | Standard combination for BPH |
| Dutasteride | Same mechanism | The combination is impractical |
| CYP3A4 inhibitors/inducers | Theoretical concentration change | Rarely has clinical significance |
| Warfarin, digoxin, theophylline | No significant interactions were found | No correction required |
| PSA analysis | Reduction by about half | Inform the doctor |
Blood donation and sports aspects
People taking finasteride are temporarily banned from donating blood. The goal is to prevent the drug from reaching the pregnant recipient. The duration of withdrawal is determined by the rules of a specific blood service; for finasteride it is usually shorter than for dutasteride, which is eliminated much longer.
It is important for athletes to know the history of finasteride status in anti-doping regulation. From 2005 to 2008, it was on the WADA Prohibited List as a masking agent because it could alter the ratio of steroid metabolites in urine.
Since 2009, finasteride has been removed from the List: improvements in analytical methods have allowed laboratories to take into account its effects. However, the Prohibited List is updated annually, so athletes should check the current edition and report all drugs in the doping control protocol.
The editors remind: information about anti-doping rules is provided for informational purposes only, and not as advice on passing control.
Editorial conclusion
The key contraindication to finasteride is pregnancy and the possibility of pregnancy, because the drug can disrupt the sexual development of a male fetus. Women should avoid contact with crushed tablets.
Caution is required in patients with liver disease, history of depression, and obstructive urinary disorders; the effect on PSA must be taken into account.
There are practically no clinically significant drug interactions with finasteride, but the combination with other 5α-reductase inhibitors is inappropriate.
For a complete picture, we recommend the articles "Finasteride side effects", "Finasteride: mechanism of action" and "Finasteride: medical indications and official use".
References
- Merck Sharp & Dohme. Proscar (finasteride 5 mg) and Propecia (finasteride 1 mg): prescribing information. U.S. Food and Drug Administration.
- Thompson IM, Goodman PJ, Tangen CM, et al. The influence of finasteride on the development of prostate cancer. N Engl J Med. 2003;349(3):215â224.
- McConnell JD, Roehrborn CG, Bautista OM, et al. The long-term effect of doxazosin, finasteride, and combination therapy on the clinical progression of benign prostatic hyperplasia. N Engl J Med. 2003;349(25):2387â2398.
- Imperato-McGinley J, Guerrero L, Gautier T, Peterson RE. Steroid 5α-reductase deficiency in man: an inherited form of male pseudohermaphroditism. Science. 1974;186(4170):1213â1215.
- Traish AM, Hassani J, Guay AT, Zitzmann M, Hansen ML. Adverse side effects of 5α-reductase inhibitors therapy: persistent diminished libido and erectile dysfunction and depression in a subset of patients. J Sex Med. 2011;8(3):872â884.
- World Anti-Doping Agency. The World Anti-Doping Code: International Standard — Prohibited List. Montreal: WADA (current edition).
