I suppose if you are concerned about growing hair on your prostate this would be a valid point.
1. To combat hair loss, propecia lowers the amount of DHT in the blood, which in turn lowers the amount of DHT in the scalp. It's lowering DHT in the scalp that's critical for hair loss. Propecia in sufficient dosages will also lower DHT in the prostate. However, saw palmetto taken orally apparently lowers DHT in the prostate but has no effect on scalp DHT at all. Thus, merely lowering DHT is not sufficient -- it's scalp DHT that is at issue for hair loss, and saw palmetto taken orally is competely useless in lowering scalp DHT.
2. It's possible that saw palmetto applied topically might have some effect on lowering scalp DHT, but whether it does is unkown. Since it doesn't cause scalp irritation and is cheap, there is no harm in including it in topical products. However, since so many people (erroneously) believe the saw palmetto is general a "DHT fighter," the biggest reason for including it in topical preparations is a marketing one. That doesn't mean it does squat, it means only that it helps sell the product. There is nothing unethical about this since saw palmetto doesn't hurt anything.
This question comes up repeatedly, even in the short time I've been here. It's a simple matter really to distinguish between fighting DHT in one part of the body and fighting it in another. What you want is to reduce scalp DHT, and taken orally, saw palmetto doesn't do that.
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A song don't have much meaning, if it don't have nothin' to say; what she could do was magic son, all I could do was play. -- Harry Chapin