A lack of visible hair growth occurs when your hair follicles continue to follow the normal hair growth cycle, but the growth is then offset by some other factor. Hair loss occurs when the hair growth cycle itself is disrupted, and comes in two forms.

In temporary hair loss, something pushes all or some hair out of the anagen, or growth, phase, and into the telogen, or shedding, phase. Hairs are cut off from nutrients, “killing” the hair, which then subsequently falls out. Once whatever caused hair follicles to shift to the telogen phase is removed, the hair growth cycle resets and normal growth recommences with no lasting damage. These types of hair loss are usually known medically as a type of “effluvium” (or outflow), such as telogen effluvium or anagen effluviem.

In permanent hair loss, the hair follicle itself is damaged, and either slowly or quickly loses the ability to grow new hair, period. Temporary hair loss can become permanent if the catalyst is not removed, and whatever interrupted the follicle slowly destroys it. Or, some problem can directly and permanently destroy the hair follicle. These types of hair loss are classed as “alopecia.”

Temporary hair loss events are actually extremely common, and usually revolve around some sudden shock to the system.

Stressful events, traumatic events, or actual literal medical shock, can all cause this type of temporary shedding. Similarly, forms of physical stress, such as weight loss exceeding 10lbs, severe illness, or surgery as well as emotional stress can cause hair fall events.

Another extremely common form of temporary hair loss in women results from sudden dramatic hormonal changes such as starting, stopping or changing hormonal birth control, pregnancy and childbirth. One of the reasons hair loss around childbirth is so common is that it is both incredibly physically stressful, emotionally stressful (especially if there are complications) and involves extreme hormone changes.

Changes in some medications, such as antidepressants, retinoids (in some acne medications), antibiotics and antifungals, anti-clotting drugs, cholesterol lowering drugs and immunosuppressants, as well as birth control can all cause hair loss. In some cases, hair will regrow once the body gets accustomed to the new normal, in other cases, hair will not regrow until the medication has been stopped, and fully left the body, which can take weeks or months.

Types of temporary hair loss that can become permanent involve more severe disruptions: some illnesses, such as thyroid disorders, scalp infections, high blood pressure, arthritis or high blood pressure, among others, and extreme medical interventions such as radiation therapy or chemotherapy. In many cases, hair regrows after the disease is treated, or in the months following radiation/chemotherapy. However, if the disorder is not diagnosed and successfully treated for a long time your scalp can become permanently damaged.

Permanent’ hair loss includes all other types of hair loss where the growth cycle is damaged. Unfortunately, this type of hair loss is the most extreme, the least understood, and the least treatable. The most common forms are alopecia areata, male pattern hair loss, and female pattern hair loss, and each are believed to have a genetic component making them more or less likely.

Alopecia areata is believed to be autoimmune, occurring when the immune system stops recognizing hair follicles as native to the body (called “immune privilege). This may occur in spots - in which case it is known as spot baldness - or it can affect the entire head or entire body.

In a minority of cases, hair regrows and the condition does not recur. In the remainder, hair follicles are gradually destroyed by the immune system, making hair loss permanent. There is no known cure, and treatments are of limited success.

MPHL is believed to be partly genetic, and partly due to male sex hormones. It cannot be cured, but, if treated early enough, can be slowed. Women with hormonal conditions such as PCOS that result in an overabundance of male hormones can also suffer from MPHL.

There are two approved treatments for MPHL. The drug finasteride can improve hair growth for androgen-dependent types of MPHL and is approved in parts of Asia, but not the US. Counterintuitively, in women with hormonal imbalances, finasteride lessens excess hair growth (hirsutism), rather than improving hair loss.

Minoxidil - known in the US by the brand name Rogaine - is approved for androgen-independent MPHL. Interestingly, minoxidil’s medical effect is to dilate small blood vessels, but it remains unclear how or why this causes hair to grow.

Both minoxidil and finasteride lose effectiveness as hair loss progresses, meaning that they are both ineffective if begun after significant hair loss has occurred, and that they cannot wholly stop hair loss. Additionally, if treatment is stopped or interrupted for a length of time, regrown hair will promptly fall out again.

FPHL is the least understood form of hair loss, with no known cause and the least number of treatment options. Finasteride has no effect on FPHL, and minoxidil can work if begun early, changes in dosage have no appreciable impact on effectiveness. As such, the only medically approved treatment for FPHL is 2% minoxidil.