As of November 2015, most reviews of natural silica or horsetail on Amazon.com indicate the product does grow hair but does not thicken it. Conversely, the German company Hübner calls upon a University Hospital of Hamburg-Eppendorf study that suggests silica does in fact increase hair thickness.
Resources at the Hüebner website indicate that its silica product is original silicea balsam, which is a pure mineral gel, dispensed and diluted with water. Meanwhile, the horsetail derived silica as sold on Amazon.com indicates that one capsule contains 500 milligrams of the horsetail herb.
The most critical reviews of silica on Amazon.com primarily pointed to consumers receiving expired product, but some 2- to 3-star reviews indicate a number of complaints of side effects such as intestinal distress and skin breakouts. The most positive reviews, in the 4- to 5-star range, suggest users should exercise patience while waiting for results and expect hair growth but not thickening.
Hüebner Original Silica Gel is also available from Amazon.com under the name Hubner Original Silica Gel, with a 15-milliliter dose containing 420 milligrams of silica. Reviews on this version of silica for hair thickening are mixed, with some users finding results after several weeks of use, and less impressed customers noting unpleasant side effects with little noticeable difference to their hair.