Botox (onabotulinumtoxinA) is made from the botulinum neurotoxin, a toxin that can cause a serious disease that leads to muscle paralysis called botulism.
However when injected directly into a muscle in approved doses, Botox prevents muscles from tightening by temporarily paralyzing them, which in turn makes wrinkles less visible.
Botox was first approved as a cosmetic by the FDA in 2002 for temporary improvement of frown lines -- wrinkles between the eyebrows, also known as glabellar lines.
Crow's feet, known scientifically as lateral canthal lines, are thought to be caused by repeated muscle contractions over a lifetime, WebMD reports. Facial expressions like frowning, smiling and squinting may leave a mark over time, is the theory.
Botox had been used for decades by opthalmologists to treat eyelid twitching, or blepharospasm, Dr. Jennifer Walden, a Texas-based aesthetic plastic surgeon, told CBSNews.com in an email. That's in part how it was discovered to smooth the skin.
While this new FDA approval for Botox only came Wednesday, Walden pointed out, "doctors have been using it off-label for years to smoothen the crow's feet."
Off-label use means a treatment is prescribed for conditions not approved by the FDA.
The new FDA approval came following a study of 833 adults conducted by the drug's manufacturer, Allergan Inc. of Irvine, Calif. where participants were randomly assigned to receive Botox or a placebo injection. People treated with Botox had greater reductions in crow's feet.
Anti-wrinkle effects from Botox typically last about four to six months.
The most common side effect seen was eyelid edema, which causes the eyelids to become swollen due to fluid buildup.
Botox labels also contain the strongest warning the FDA offers -- a boxed warning -- that says the effects of the toxin may spread from the area of injection to other areas of the body, causing symptoms similar to those of botulism. The FDA notes though there's never been a confirmed serious case of this happening when Botox is used at recommended doses.
Aesthetic, or cosmetic, medicine physicians have felt comfortable about the drug's safety, which is why Botox has been frequently used for crow's feet and other unapproved areas of the face like the side of the nose, explained Dr. Marco Harmaty, a plastic and reconstructive surgeon at The Mount Sinai Medical Center in New York City.
The FDA's new approval "does give you an added benefit and safety of saying that I'm not doing anything illegal or potentially harmful," he said to CBSNews.com.
Competitors to Botox include Medicis' Dysport and Merz Aesthetics, Inc's Xeomin. These other drugs are approved by the FDA for frown lines, but have not yet been approved by the agency for crow's feet, which may if anything give Botox a marketing edge, Harmaty speculated.
Botox for cosmetic purposes still likely won't be covered by insurers, given it is used for cosmetic purposes. Insurers often cover people who have approved medical conditions treated by Botox, including chronic migraines, severe underarm sweating, blepharospasm and strabismus, when both eyes are misaligned.
"I don't necessarily foresee (the new drug approval) changing people's practices too much,' said Harmaty.