Why Hair Falls Out: The Main Causes

When a person's hair falls out, it's common to ask why. The reasons are varied and determine the treatment to be applied. Learn more.
Why Hair Falls Out: The Main Causes
Diego Pereira

Written and verified by el médico Diego Pereira.

Last update: 23 April, 2023

It’s clear that if we talk about why hair falls out (alopecia), we’re facing one of the most important aesthetic problems in the world. Until recently, therapeutic measures were very limited, although more options are now possible.

Of course, this depends on the cause of the alopecia, for which you’ll need to consult with a dermatologist. For guidance purposes, we’ve prepared the following article to clarify some doubts.

Why hair falls out

The answer to this question is multifactorial, depending on the age and particular characteristics of each patient. In general terms, the most frequent non-pathological cause of alopecia involves hormonal factors, as occurs in androgenic alopecia. This is responsible for hair loss in most men and occurs any time after puberty.

For practical purposes, we’ve divided the causes of alopecia into two: Scarring and non-scarring. The first includes irreversible damage to the hair follicle and in the second, the damage is more localized, so it could be reversible.

What are hair follicles like?

At a microscopic level, the hair follicles are the structure responsible for the creation and growth of hair. They’re characterized by having stem cells inside, which are capable of generating new tissue, which is why hair grows after being cut.

In addition, there are associated sebaceous glands that are responsible for the production of sebum. This substance is intended to lubricate each hair and prevent damage caused by external agents.

If this follicle doesn’t exist, regardless of the cause, there can be no hair growth. That’s why the application of topical medications (such as minoxidil) is only effective if the structure is preserved.

A man who's starting to lose hair.
In men, the hormonal role is clearer and is also associated with age.

Non-scarring alopecia

In this case, hair falls out due to internal or external causes to the body without permanent damage to the hair follicle. The most well-known examples are androgenic alopecia, hair loss caused by the application of chemical products, alopecia areata, and ionizing radiation.

Androgenic alopecia

This is the most frequent type of non-scarring alopecia. The term androgenic refers to the effect of male hormones, such as testosterone. Virtually all men will suffer from this condition, with the consequent emotional problems in some cases.

Generally, hair loss begins at the frontal portions and spreads symmetrically towards the occipital region, almost always leaving traces of hair towards the lateral portions of the head.

Despite the fact that it affects a large part of the population, the medications that can stop its evolution are limited. These include minoxidil, finasteride, and some antioxidants taken orally. In addition, there’s a surgical treatment that’s gaining popularity.

This condition also affects women, especially after menopause, when androgen levels increase, causing symptoms similar to those of men, but with a different distribution.

Hair loss due to the application of chemicals

It’s logical to think that human hair isn’t prepared to withstand chemical products. Dyes can cause the weakening of the hair and the hair follicle.

This produces constant changes, both in the natural lubrication and in the keratin around the follicle. The most direct consequences are the progressive decline in growth and frequent ruptures.

Alopecia areata

This is also one of the most common forms and can affect both men and women. Its hallmark is regular, well-defined patchy hair loss.

It can occur in isolation or in patches, as well as adopting different styles depending on the location. There’s no underlying inflammatory phenomenon, but the hair simply falls out with very mild efforts. Although its cause isn’t known, it’s related to autoimmune processes.

Diseases such as vitiligo or some types of thyroiditis are often associated with this disease. In fact, it’s a red flag for dermatologists to look for possible associated disorders.

Ionizing radiation

There are endovascular neurosurgical procedures that require the use of radiation to image the intracranial blood vessels. This allows the doctor to cure diseases, such as arteriovenous malformations.

A small percentage of patients, regardless of age, may develop a picture of transient alopecia. They usually recover on their own after a few weeks.

Scarring alopecia

Unlike the previous group, this type of alopecia is characterized by irreversible damage to the hair follicle. In this case, the skin looks shiny, smooth, and with a little redness. We’ll describe injuries and burns, bacterial dermatoses, fungal infections, tumors, and chronic discoid lupus erythematosus.

Trauma and burns

These are common causes and tend to leave scar tissue around the injury. It’s highly unlikely that hair will grow back after a traumatic event, with surgical treatment being an alternative.

It’s important to clarify that local procedures such as cryosurgery, radiotherapy, and implants are also considered microtrauma. These can lead to the appearance of erosive pustular dermatosis, a disorder that can lead to patches of alopecia.

In this condition, pustules (lesions with pus) are produced without the presence of causative microorganisms. When they come together, they can cause the destruction of the hair follicle.

Bacterial dermatoses

Many bacterial infections give rise to extensive inflammatory processes. Staphylococcus aureus is the most involved microorganism and gives rise to a condition known as folliculitis.

This term refers to the localized infection of a hair follicle, the associated inflammation of which affects its function. When the lesion spreads, a condition called furunculosis may form, which leaves sequelae. Some risk factors for this disease include constant stress, immunosuppression, and some forms of acne.


This term refers to a fungal infection. When it affects the skin and subcutaneous tissue, it’s considered “superficial”. There are many types, but the one most associated with hair loss is tinea capitis .

This is classified within the group of dermatophytosis (diseases caused by fungi that affect keratin) and is characterized by itching and flaking of the skin. While there are much more severe inflammatory varieties, there’s also a disease known as Celsus’s kerion. It produces pus over a large area of the head with scabs that, when removed, leave almost permanent alopecia.


Cysts and neurofibromas are some of the benign tumors that can leave small patches of permanent alopecia. Others, considered malignant (such as basal cell epithelioma) also produce this symptom, although to a lesser extent.

Finally, metastatic lesions can induce inflammatory skin reactions and cause alopecia. This means that a malignant tumor exists elsewhere in the body and has spread to the scalp.

A man's hair falls out in patches.
Patchy hair loss can be infectious, due to head fungus.

Chronic discoid lupus erythematosus

Lupus is an autoimmune disease that affects a large number of organs. This variety of the disease affects the hair, forming patches with irregular edges and reddened skin.

Lesions appear in young women, who are the most likely to develop the disease. The image is usually very characteristic, which is why it’s a sign that alerts dermatologists and forces them to rule out its presence. It can leave areas permanently bald, although others respond to topical medications.

Is there treatment when hair falls out?

The treatment of alopecia depends greatly on the cause. Some of the approaches are as follows:

  • Oral route: Antioxidants, vitamins, and finasteride
  • Topical route: Minoxidil and steroids
  • Surgical: Scalp transplantation

The last option has gained a lot of fame in recent years due to the excellent results in the medium term. For example, it allows hair to grow in areas that aren’t sensitive to drugs, as occurs in androgenic alopecia. Its disadvantages include high cost and long duration, despite being an outpatient treatment.

  • Modrego L, Barberá F. Alopecias: orientación diagnóstica, clínica y terapéutica. Medicina Integral 2000;35(2):55-71.
  • Suro J, Bouhanna P. Transplante de pelo, cirugía menor de larga duración. Dermatología Cosmética, Médica y Quirúrgica 2008;6(2):126-130.
  • Guzmán-Sánchez D. Alopecia androgenética. Dermatol Rev Mex 2015;59:387-394.
  • Álvarez, Israel Sánchez, Wendy Carolina González Hernández, and Rosa María Ponce Olivera. “Inmunología de la alopecia areata. Pérdida del privilegio inmunológico (parte I).” Dermatología Cosmética, Médica y Quirúrgica 16.3 (2018): 229-236.
  • Llancapi, Patricio, Carolina Delgado, and Francisco Mendoza. “Pustulosis aguda erosiva del cuero cabelludo.” Revista Chilena de Dermatología 32.3 (2018).
  • Gallegos-Ríos, Martín Alejandro, Itzel Alejandra Martínez-Magaña, and Conrado Romo-Sánchez. “Discoid lupus erythematosus.” Dermatología Revista Mexicana 60.3 (2016): 253-256.
  • Salduna, María Dolores, et al. “Tiña capitis.” Dermatología Argentina 24.4 (2018): 194-198.
  • Ramos, Emilio Villodres, Cristian Fischer, and Jose Marıa Mir Bonafe. “Novedades en el trasplante de pelo.” Piel 1388 (2017): 1-10.

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