Anagen refers to the growth phase of the hair growth cycle in humans. It can last anywhere from 2 to 6 years (or more) and include up to 90% of the hair on the head. The telogen phase it the rest phase for hair, which usually lasts just a few months. After this rest phase is over, the hair that was resting is shed and replaced by new hair. We usually lose between 100 to 150 hairs a day under normal circumstances. Anagen effluvium is hair that is lost during the growth phase - the time when the hair is especially sensitive to anything that interferes with the function of its follicles.

Anagen effluvium results in tapered and weakened hair shafts that are easily broken. Hairs grow by rapid cell division in the shaft, which causes the hair fiber to grow at a rate of about ½" a month. With this disorder, the rapidly dividing cells are injured sufficiently to make them prone to breakage and fallout. The hair tends to break off at the scalp level rather than being shed as it normally does. Any injury to developing hair shafts can trigger anagen effluvium.

The most common causes of this kind of hair loss are chemotherapy and radiation therapy. Chemotherapy drugs attack rapidly growing cells (cancer cells) but can also assail other rapidly dividing cells, such as hair cells. These drugs can severely impact the formation of hairs and in some cases; completely interrupt the hair growth cycle. This type of trauma only happens with those hairs in the anagen growth phase, and usually occurs 1 to 3 weeks after treatment begins. This is generally a temporary condition as the hair almost always grow back, sometimes healthier and thicker than before. Another name for anagen effluvium is toxic alopecia.

Radiation therapy may also cause anagen effluvium in a manner similar to the way chemotherapy causes it. The degree to which the hair is affected is usually related to the dose the patient received. The hair is weakened in the same way it is with drug cancer therapy in that the hair tapers to a weakened segment, that's prone to breakage. Like chemotherapy, the hair returns to normal health after treatment is concluded.

There are other substances that are toxic to humans that can cause this disorder. A number of poisons such as arsenic, thallium and rat poison can result in the rapid loss of hair. Any substance that affects the ability of rapidly growing cells to divide has the potential for causing anagen effluvium. Sometimes the hair is so severely affected that it can be pulled out in large clumps.

Treatment for anagen effluvium is dependent upon cessation of the cause of hair loss. If either chemotherapy or radiation therapy is the reason for it, then the loss will stop and hair will be restored when the therapy is completed. But if it is due to the ingestion of some other substance, the causative substance must be determined before hair can be restored.