A man's mustache grows just like scalp hair--from hair follicles situated in your skin. Some men grow a mustache that's thick and full, while others' are sparse and thin, just like the variations of scalp hair from one person to the next. Illness, injury and hormonal imbalance can all cause you to suffer from mustache hair loss; in most cases your facial hair will grow back after the underlying cause is addressed.
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Hair loss due to cancer treatments, such as radiation and chemotherapy, is a widely-known side effect. According to the American Cancer Society, losing your hair from the potent drugs is not limited to scalp hair. People of both genders who undergo treatment for cancer may find that body hair, including a man's mustache, can also fall out during the course of treatment. In most cases, hair regrowth begins after chemotherapy and radiation has been completed.
Alopecia Areata Universalis
Alopecia areata universalis is an auto-immune condition in which you lose hair on your scalp and your body, including facial hair. Your immune system essentially attacks itself, and de-activates functioning hair follicles, which leads to hair loss. Medications and other treatments may help reduce the inflammation in your hair follicles, which allows them to function properly again and grow hair. According to the National Alopecia Areata Foundation, alopecia areata universalis is not as common as other forms of alopecia that affect only scalp hair.
Men who have low testosterone levels due to a medical condition called hypogonadism--also called testicular failure--may lose mustache hair or are unable to grow facial hair in the first place. The National Institutes of Health MedlinePlus website explains that testicular failure can be the result of illness, injury or medication use. Symptoms can include a regression of or delayed onset of secondary sex characteristics, such as facial, armpit and pubic hair, infertility and lack of muscle development. Testosterone therapy may be a solution for men who have low hormone levels that affect their mustaches.