| The term ''natural'' can be confusing when used in connection with hormones. When we say a hormone is ''natural,'' we are referring to the molecules that make up the structure of the hormone. A natural hormone has a chemical structure that is identical to the hormone naturally produced by the body, which is why natural hormones are also called ''natural'' hormones. A ''natural'' hormone does not mean that it is a product purchased in a health food store. The important thing to remember is that for a hormone to be considered ''natural'' or ''natural,'' its structure must replicate exactly the structure of the hormones your body produces. On the other hand, ''natural'' and ''synthetic'' hormones should not be considered the same or used interchangeably. A ''synthetic'' hormone has a chemical structure that may be similar to, but is not exactly the same as, a hormone produced by your body. These chemical differences can mean that the synthetic hormone acts differently in your body and produces substantially different effects. For example, let's look at the difference between the synthetic progestins and natural progesterone. The synthetic progestins, such as commonly prescribed Provera, are similar to the progesterone your body produces, but the subtle chemical differences can significantly influence the hormone's action and side effects in the body. Synthetic progestins can cause side effects of irritability, nausea, depression, and water retention in some women. Natural progesterone is identical to the hormone made in the body, and many women find it easier to tolerate. 
You can see that the synthetic hormone medroxyprogesterone acetate, commonly known as Provera? (bottom), has a different chemical stucture than natural progesterone (top) | 
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