As with any other medicine, Propecia too comes with an array of possible side effects that might manifest in a small number of people using this treatment. In some cases, these adverse effects will simply go away on their own after a short period of time, while for others they might last over the course of the whole treatment. In either case, you should notify your doctor as soon as you notice anything that is out of the ordinary.

What’s on this page?

The most dangerous side effect of all medicines is an allergic reaction. Reactions to different medicines can be very severe and despite being very rare in case of Propecia, if they do appear, you should seek immediate medical attention. The tell-tale signs of an allergic reaction to Propecia’s active ingredient finasteride include:

  • Swelling of the throat, face or lips
  • Difficulty breathing or swallowing
  • Hives
  • Itchy rash

Other very rare, yet rather worrying side effects that might require you to immediately contact a medical professional include mood changes on one hand, and changes in the breast tissue on the other:

  • Depression
  • Suicidal thoughts
  • Unusual mood changes and mood swings
  • Breast pain
  • Breast lumps
  • Enlargement of the nipples
  • Nipple discharge

Aside from these side effects that can result in termination of the hair loss treatment, below we will discuss other adverse effects that affect a certain number of Propecia users, but which usually aren’t as dangerous as those we mentioned above.

What do the clinical trials say?

When Propecia was tested prior to its release, a double blind, placebo-controlled study has been conducted, spanning the period of one year and including as much as 3.200 patients. The safety profile of Propecia, according to this study, is rather similar to that of placebo, making it exceptionally safe. Over the course of the experiment, the treatment was stopped because of “clinical adverse experience” in 1.7% of 945 men who were taking Propecia. In comparison, the experiment was stopped due to very same reason for 2.1% of 934 men who were taking the placebo.

Propecia and sexual desire or function

Due to the fact that Propecia works by interfering with the hormonal processes in the body, it’s not surprising that a number of documented side effects are closely related to sexual desire and function. These adverse effects are listed as uncommon in the official patient information leaflet, meaning that they are observed in one in hundred people. They include:

  • Impotence (erectile dysfunction)
  • Decreased libido
  • Problems with ejaculation (most often, producing a very small amount of semen)

While for most people these side effects will disappear once the treatment is terminated, it seems that for an unspecified percentage, all three issues can persist for longer period of time after Propecia isn’t used anymore.

Returning to the findings of the double blind experiment conducted by the manufacturer of Propecia, Merck Sharp & Dohme, the exact data on side effects of sexual nature can be presented as follows:

Side effect Percentage of men who reported side effects (using Propecia) Percentage of men who reported side effects (using Placebo)
Decreased libido 1.8% 1.3%
Erectile dysfunction 1.3% 0.7%
Decreased volume of ejaculate 0.8% 0.4%

In the first year of the trial, side effects related to sexual desire and function were reported by 3.8% of the subjects administered Propecia and 2.1% of subjects in the test group. However, over the course of the following three years, the prevalence of these issues in the treatment group decreased down to 0.6%.

In line with this, but also with later experiences reported by different men who used Propecia, it would seem that side effects related to sexual function are pronounced, but mostly during the first year of the treatment, after which they begin to diminish.

However, it is important to underline that in some cases, these side effects can persist long after the cessation of the treatment.

Propecia and fertility

According to the official data provided by the MSD, the pharmaceutical company behind Propecia, male infertility and problems with sperm quality are considered to be side effects of this medicine with an unknown frequency.

However, the clinical trials provide us with no information on this question – all the men who said they are planning to gather a child were removed from the trials prior to their start. On the other hand, studies conducted on animals failed to produce statistically significant information regarding this topic.

So, all the reports about the adverse effects when it comes to male fertility were made only after Propecia was released on the market. Retrospective examination of those claims showed that a significant percentage of those affected had other potential risk factors, which makes it increasingly difficult to determine with certainty whether this side effect is really present.

As of now, there are no conclusive evidences, but there are some indication that Propecia does reduce the quality and the number of sperm cells in the semen. At the same time, these issues will bring back to pre-treatment levels once the treatment is terminated. Despite this, studies into the potential long-term impact on male fertility have yet to be conducted.

Can Propecia cause breast cancer?

One of the side effects that should be immediately reported to the doctor is abnormal changes in the breast tissue. These can include lumps, nipple discharge, enlargement of the breast tissue or pain and over-sensitivity.

However, this potential side effect has not been documented in the clinical trials and has only become known once after Propecia was released on the market. So far, studies aimed at calculating the prevalence of breast cancer in the users of Propecia have yet to produce any significant results.

Mood changes and mental health issues

Among the side effects classified as uncommon there are multiple references to potential mental health issues and mood changes. Even the official patient leaflet advises the users of Propecia to immediately contact their doctor if they start noticing symptoms of depression, changes in mood or suicidal thoughts. However, these effects have also been absent from the original clinical trials and were only subsequently reported by the users.

I have noticed side effects not listed in the patient leaflet, what should I do?

If you notice any adverse effects that have not been mentioned in the official patient information leaflet, you can report them through the Yellow Card Scheme, thus helping MHRA to look deeper into them and potentially update the safety information.1 Also keep in mind that side effects of branded Propecia will coincide with the generic versions of this hair loss medicine.


  1. Information – Yellow Card Scheme