The warning letters advise sellers of the products that their marketing claims may be illegal and that health-related claims must be supported by scientific evidence.
Another 16 sellers received letters from the US Food and Drug Administration (FDA), warning them that their business practices may violate FDA law.
The websites were identified during an FTC Internet surf of sites making claims that their hormone replacement therapy alternative products - for example, progesterone creams, sprays or dietary supplements containing plant-based hormones - could cure diseases or prevent them.
The letters note that the FTC staff is not aware of any competent and reliable scientific evidence to support claims that the types of products advertised could prevent, treat, or cure cancer, heart disease, or other diseases, prevent osteoporosis, or increase bone density.
They also emphasize that according to FTC case law, all health claims - including claims about the safety of natural hormones - must be supported by reliable scientific evidence.
The FTC has launched a similar campaign against makers of products that allege false weight loss claims and in recent months it has extracted hefty penalties from some companies found to have breached its regulations.
A survey by the organisation revealed earlier this year that the number of obviously false weight-loss claims for dietary supplements, creams and patches had dropped from 50 percent in 2001 to just 15 percent in 2004, suggesting that its methods may have some effect.