Hot flashes (also known as hot flushes, or night sweats if they happen at night) are a symptom which may have several other causes, but which is often caused by the changing hormone levels that are characteristic of menopause.
Hot flashes, a common symptom of menopause and perimenopause, are typically experienced as a feeling of intense heat with sweating and rapid heartbeat, and may typically last from two to thirty minutes for each occurrence. The sensation of heat usually begins in the face or chest, although it may appear elsewhere such as the back of the neck, and it can spread throughout the whole body. Some women feel as if they are going to faint. In addition to being an internal sensation, the surface of the skin, especially on the face, becomes hot to the touch. This is the origin of the alternative term "hot flush," since the sensation of heat is often accompanied by visible reddening of the face. Excessive flushing can lead to rosacea.
The hot-flash event may be repeated a few times each week or every few minutes throughout the day. Hot flashes may begin to appear several years before menopause starts and last for years afterwards. Some women undergoing menopause never have hot flashes. Others have mild or infrequent flashes. The worst sufferers experience dozens of hot flashes each day. In addition, hot flashes are often more frequent and more intense during hot weather or in an overheated room, the surrounding heat apparently making the hot flashes themselves both more probable and more severe.
Severe hot flashes can make it difficult to get a full night's sleep (often characterized as insomnia), which in turn can affect mood, impair concentration, and cause other physical problems. When hot flashes occur at night, they are called "night sweats." As estrogen is typically lowest at night, some women get night sweats without having any hot flashes during the daytime.
Types of hot flashes
Some menopausal women may experience both standard hot flashes and a second type sometimes referred to as "slow hot flashes" or "ember flashes." The standard hot flash comes on rapidly, sometimes reaching maximum intensity in as little as a minute. It lasts at full intensity for only a few minutes before gradually fading.
Slow "ember" flashes appear almost as quickly but are less intense and last for around half an hour. Women who experience them may undergo them year-round, rather than primarily in the summer, and ember flashes may linger for years after the more intense hot flashes have passed.
In younger women
If hot flashes occur at other times in a young woman's menstrual cycle, then it might be a symptom of a problem with her pituitary gland; seeing a doctor is highly recommended. In younger women who are surgically menopausal, hot flashes are generally more intense than in older women, and they may last until natural age at menopause.
Hormone replacement therapy
Hormone replacement therapy may relieve many of the symptoms of menopause. However, HRT may increase the risk of breast cancer, stroke, and dementia and has other potentially serious short-term and long-term risks The U.S. FDA and women's health advocates recommend that women who experience troublesome hot flashes try alternatives to hormonal therapies as the first line of treatment. If a woman chooses hormones, they suggest she take the lowest dose that alleviates her symptoms for as short a time as possible.
Selective estrogen receptor modulators
SERMs are a category of drugs that act selectively as agonists or antagonists on the estrogen receptors throughout the body. Tamoxifen, a drug used in the treatment of some types of breast cancer and which can cause hot flashes as a side effect, RAD1901, Raloxifene and the soy-derived Femarelle (DT56a) are examples of SERMs. Menerba, a botanically derived selective estrogen receptor beta agonist currently under development, works like a SERM, but only activates on the estrogen receptor beta. 
Selective serotonin reuptake inhibitors
SSRIs are a class of antidepressants most commonly used in the treatment of depression, and some personality disorders. They have been found as efficient in alleviating hot flashes.
Isoflavones are commonly found in legumes such as soy and red clover. The two soy isoflavones implicated in relieving menopausal symptoms are genistein and daidzein, and are also known as phytoestrogens. The half life of these molecules is about eight hours, which might explain why some studies have not consistently shown effectiveness of soy products for menopausal symptoms. Although red clover (Trifolium pratense) contains isoflavones similar to soy, the effectiveness of this herb for menopausal symptoms at relatively low concentrations points to a different mechanism of action.
It is believed that dietary changes that include a higher consumption of phytoestrogens from sources such as soy, red clover, ginseng, and yam may relieve hot flashes.
Ginseng Very few studies exist on the effect of ginseng for relief of menopausal symptoms. In a large double-blinded randomized controlled trial, reduction in hot flashes was not statistically significant but showed a strong trend towards improvement.
Flaxseed There have also been several clinical trials using flaxseed. Flaxseed is the richest source of lignans, which is one of three major classes of phytoestrogen. Lignans are thought to have estrogen agonist and antagonist effects as well as antioxidant properties. Flaxseed and its lignans may have potent anti-estrogenic effects on estrogen receptor positive breast cancer and may have benefits in breast cancer prevention efforts. One recent study done in France, looked at four types of lignans, including that found in flaxseed (Secoisolariciresinol) in a prospective cohort study to see if intake predicted breast cancer incidence. The authors report lowered risk of breast cancer among over 58,000 postmenopausal women who had the third highest quartile of lignan intake. There have been a few small pilot studies that have tested the effect of flaxseed on hot flashes. Currently there is a large study sponsored by the National Cancer Institute that is ongoing, but not accepting any new participantshttp://clinicaltrials.gov/ct2/show/NCT00956813. The rationale for the study is that estrogen can relieve the symptoms of menopause, but can also cause the growth of breast cancer cells. Flaxseed may reduce the number of hot flashes and improve mood and quality of life in postmenopausal women not receiving estrogen therapy.
Lifestyle changes may help alleviate hot flashes. These include avoiding caffeine, hot drinks, chocolate, spicy or hot foods and alcohol.
Hot flashes in men could have various causes. One is a possible sign of low testosterone or embarrassment. Another is andropause, or "male menopause."  Men with prostate cancer or testicular cancer can also have hot flashes, especially those who are undergoing hormone therapy with antiandrogens, also known as androgen antagonists, which reduce testosterone to castrate levels. There are also other ailments and even dietary changes which can cause it  Men who are castrated can also get hot flashes.
It has been speculated that hot flashes are considerably less common among Asian women, possibly owing to their soy rich diets.
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