Minoxidil (brand name Rogaine) is known to be one of only two medications officially approved for hair loss treatment (the other being Finasteride). Minoxidil was first approved by the US FDA to treat hair loss in men in 1988. In 1991 the product was also made available for women with hair loss. Make sure to read my post on whether Minoxidil can grow a beard.

Minoxidil Chemical Structure Also see some before and after results of patients on Minoxidil and/or Finasteride.

This post was originally written in 2014. It has now been updated with newer studies discussing Minoxidil’s mechanism of action in stimulating hair growth.

How does Minoxidil Work?

Scientists do not know the exact mechanism via which Minoxidil® has a positive effect on hair growth. However, there are a number of proven mechanisms of action that suggests how Minoxidil stimulates hair growth.

The original use of Minoxidil was as an oral medication for high blood pressure. The side effect of hypertrichosis (excessive body hair) led to its becoming a popular treatment option for hair loss.

To date, the main hypothesis about how Minoxidil works relates to its vasodilatory, potassium channel opening and increased blood flow effects. There are also other theories about how Minoxidil helps grow scalp hair, and further below I have outlined all of the main ones.

It should be noted that Minoxidil, besides prolonging the growth phase of the hair cycle, has also been shows to increase the diameter of existing hair follicles. According to a study from 1988, seven subjects who received a 5 percent dose of minoxidil had a mean hair shaft diameter of 0.029 mm before treatment, which then increased to 0.043 mm at 12 weeks.

Minoxidil and Hair Growth Simulation Studies

In 1997, researchers found that Minoxidil increased prostaglandin synthesis (more specifically, prostaglandin synthase-1, abbreviated as PGHS-1) in cultured dermal papilla cells. In more recent years, the issue of prostaglandins and hair loss has garnered a great deal of attention and you can search for “PGE2” on this blog to learn more.
A French study from 1998 is among many that has found that Minoxidil upregulates growth factors, in particular vascular endothelial growth factor (VEGF).
A 2001 study found that the positive effect of Minoxidil on hair is mediated by adenosine.
An excellent article from 2008 on hair loss medical treatments by Dr. Nicole Rogers and Dr. Marc Avram that discusses Minoxidil in detail. They mention that one of the main effects of Minoxidil is angiogenesis and increased blood flow in the area of application. They also discuss the enhanced cell proliferation and DNA synthesis effects on Minoxidil that might be benefiting hair growth.
In 2011, South Korean researchers found that Minoxidil activated the β-catenin pathway in human dermal papilla cells and therefore extended the anagen (growth) phase of the hair cycle.
In April 2014, Taiwanese researchers came up with yet another reason as to why Minoxidil works, concluding that Minoxidil may suppress androgen receptor-related functions. i.e., the drug has anti-androgenic properties. Their conclusion is especially interesting:

The current findings provide evidence that minoxidil could be used to treat both cancer and age-related disease, and open a new avenue for applications of minoxidil in treating androgen-AR pathway-related diseases.

In January 2017, Turkish scientists concluded that Minoxidil acts as an antiandrogen.
In February 2017, an unrelated study from Japan had this to say about Minoxidil:

Minoxidil enhances hair keratinocyte proliferation and activates hDP cells to induce growth factors. IGF-1 is among these growth factors, and has been shown to exhibit a potent hair elongation effect.

In June 2017, US researchers published a new study that provided significant insights into how minoxidil foam worked via upregulating and downregulating various genes. Interestingly, vertex and frontal scalp of patients showed a generally similar response to minoxidil. Many online reports suggest that minoxidil might work better in the crown than in the front, but perhaps that is not true based on these findings.
In February 2018, South Korean scientists discovered that Minoxidil promotes hair growth through the stimulation of growth factor release from adipose-derived stem cells. This growth factor secretion may enhance hair growth by promoting dermal papilla cell proliferation.

Minoxidil Side Effects

In general, topical Minoxidil is well tolerated in most people at the typical 5% dosage. Most people even tolerate higher concentration levels of the drug. However, some people will get side effects.

The most common entail adverse skin reactions such as burning, itching, redness and stinging in the areas of application. Another common complaint is an increase in body hair growth after taking Minoxidil, especially in the forehead, eyebrow and beard regions.

In rare instances, people complain about dizziness or breathing difficulties after taking Minoxidil. Allergic reactions, including rashes, are also possible in some cases. Please see a doctor immediately if you get such serious side effects. Also make sure to stop using this medication right away.

Minoxidil Shedding

Some people will shed a lot of hair after changing their Minoxidil® dosage. For those who quit Minoxidil entirely, a major shed of scalp hair is almost always guaranteed. Sometimes this can take weeks or even months after drug use cessation. In many instances, sheds are temporary and just the regular part of the hair cycle.

Minoxidil Toxicity in Cats

If you own pets, note that Minoxidil® is very poisonous to some animals, especially cats. If your cat is exposed to Minoxidil via a spill or accident, some side effects to look out for include:

Fatigue and lethargy.
Changes in heart rate due to cardiac damage.
Dehydration.
A drop blood pressure (hypotension).
Coughing.
Changes in appetite.

Prompt action and treatment by a veterinarian will prevent your cat from dying. If the medication was applied topically, make sure to wash the cat’s paws and fur promptly and thoroughly.

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