The Anagen Phase of Hair Growth

The anagen phase is the active growth phase of hair follicles. During this first of three hair phases, a strand can grow about one centimeter every 28 days. The duration of the anagen phase differs for everyone and is largely what dictates how long you can grow your hair.

A number of things, including environmental or genetic factors, can disturb anagen hair growth, resulting in hair loss or the inability to grow long hair.

This article discusses what happens during the anagen phase of hair growth and factors that can affect hair in this growth stage.

What Happens During the Anagen Phase?

During the anagen phase, the cells in the root of the hair divide rapidly, adding to the hair shaft. Scalp hair stays in this active phase of growth for two to six years. At any time, about 90% of the hairs on your head are in the anagen phase.

The amount of time that a hair follicle stays in the anagen phase is genetically determined. Some people naturally have longer anagen phases and can grow their hair very long, while others will never see their hair get much longer than a foot and a half.

An unknown signal causes the end of the anagen phase.

After the Anagen Phase

After this, your hair goes through two more phases—catagen and telogen—before cycling back to the anagen phase.

It's important to note that all hairs do not go through these stages at the same time. The reason that you don't temporarily go bald is that, at any given moment, some hairs are in the anagen phase, some hairs are in the catagen phase, and some hairs are in the telogen phase. 

Phase 2: Catagen

The catagen phase is a short transition stage that occurs at the end of the anagen phase. It signals the end of the active growth of a hair.

The hair detaches from its blood supply during the catagen phase. The outer root sheath then shrinks and attaches to the root of the hair, forming a club hair. This phase lasts for about two to three weeks.

Phase 3: Telogen

After the short catagen phase, the hair is released and the hair follicle rests for three months. The club hair falls out. Typically, you lose 50 to 100 hairs per day. After three months, the follicle goes back into the anagen phase and begins to grow a new hair.

Some experts refer to exogen, release of the hair shaft, as a fourth phase.

What Can Shorten the Anagen Phase?

A decrease in the number of hair follicles growing hair can cause more hair follicles to go into the telogen phase at the same time. This results in less time spent in the anagen phase overall and a type of diffuse hair loss known as telogen effluvium. Those with the condition may have hair that is noticeably thin.

Common causes of telogen effluvium include:

• High fever caused by an illness
• Childbirth
  
• Surgery
• Severe emotional stress
• Poor nutrition
• Significant weight loss
• Certain medications (including beta blockers, anticoagulants, and antidepressants)
• Stopping hormonal birth control

Anagen effluvium is a form of hair loss that occurs due to injury of the hair follicle during the anagen phase. Chemotherapy is a common cause.

Other causes of anagen effluvium include:

• Radiation
• Toxic heavy metals
• Toxic chemicals
• Inflammatory disease

These disrupt the hair while it is in the growth phase, leading to diffuse and often abrupt hair loss. In such cases, the hair will usually recover to its prior fullness once the offending agent is stopped.

Related Conditions

Short anagen syndrome and loose anagen syndrome are two conditions that can affect this phase of hair growth, irrespective of the causes listed above.

Short Anagen Syndrome

Short anagen syndrome is a rare condition that shortens the anagen phase. People with this syndrome say that their hair does not grow and/or they have never needed a haircut

An exam will reveal a predominance of telogen hairs. The cause of this syndrome is unknown.

Loose Anagen Syndrome

Loose anagen syndrome is seen in some children. Hair is sparse, and can easily and painlessly pulled out. If it is, the roots show that hair is in the anagen phase.

This condition is caused by a defect in the way the hair is attached to the scalp. It may be an inherited condition and it usually improves as the child ages.

Can Anagen Stimulators Help?

Some hair products claim to be "anagen stimulators" that either induce hairs to go into the anagen phase or help hairs stay in the anagen phase longer.

Rogaine (minoxidil) is a topical medication that stimulates hair growth by:

1. Causing telogen hair follicles to enter into the anagen phase
2. Lengthening the anagen phase

Although this medication is FDA-approved to treat male- or female-pattern baldness, there is still no proof of its efficacy in treating other hair loss conditions associated with the anagen phase.

That said, there is also little evidence to support the use of many other types of hair growth products.

Before using any over-the-counter products, first talk to a healthcare professional—ideally, a dermatologist (a physician who specializes in treating hair, skin, and nails).