What They Are:

While silicone breast implants are sacks containing silicone gel, liquid silicone injections are just that: loose silicone inserted beneath the skin (subcutaneously) by a needle. Liquid silicone injections are not approved means of breast augmentation in the United States, because they are more dangerous and less effective than implants.

History:

In the 1940s, women began receiving liquid silicone oil injections, a disastrous practice that led to the silicone spreading throughout the body and causing capsular contracture; where the body walls off a foreign substance behind internal scar tissue. Regardless, people tried to upgrade the injectable silicone and continued using them through the 1960s.

In 1992 the FDA banned liquid silicone injections, along with silicone breast implants; citing the numerous harmful side effects, especially concerns that the silicone caused cancer and other diseases.

Later in the 1990s, the FDA approved a new, purified-silicone injectable for limited use; not including cosmetic alteration.

However, neither this ban, nor the countless cases of complications and even death resulting from silicone injections has stopped people from using them.

Black Market Procedures:

Because liquid silicone is illegal for cosmetic procedures, patients find the injections through the black market, either in other countries, (from practitioners posing as doctors), or from doctors willing to risk losing their medical license if something goes wrong.

Occasionally, women will receive illicit injections in their buttocks, but those who want larger breasts tend to stick with legalized implants. Injections have mostly been relegated to transvestites and men looking to increase penis size.

Underground injections are especially risky because there is no way to know whether the silicone being used is medical-grade, or simply bought from a local hardware store. Lower-quality silicone will lead to even more complications.

Silicone Migration: Why Are Injections Still Illegal, But Not Silicone Implants? 

Short answer: Silicone implants leak. Since the implant begins as a single mass, the body walls it off in a capsule of scar tissue. Silicone injections do not start as one single mass, and thus migrate sooner and further, causing more complications, including scarring and disfiguration. Silicone implants returned to the market in 2006, 14 years after the 1992 ban, aided by numerous studies proving the safety of newer versions developed in the 1990s. Injection never saw the same luck, largely because it had no advantage over implants, so medical companies had no desire to fund research or development.

At all costs, avoid silicone injectables. Anyone offering them is inviting you to undergo an unapproved, dangerous procedure.