Eflornithine hydrochloride (sold under the brand name Vaniqa) is a prescription cream applied to the skin for the reduction of unwanted facial hair in women ages 12 and older. For unknown reasons, Vaniqa does not work for everyone.
Hair growth cells and cancer cells share some interesting characteristics: rapidly dividing with multiple potentials for differentiation. This is part of the reason chemotherapy and radiation can result in hair loss: they disrupt the same kinds of cellular activity.  It has been theorized that some cancer drugs may be used to induce a controlled amount of hair loss or reduction.Vaniqa's active ingredient, eflornithine hydrochloride, has been observed to result in antitumor activity.  It is the first commercially-available topical preparation to come out of this sort of research.
Vaniqa became available on July 31, 2000. Originally made by Bristol-Myers Squibb in a partnership with Gillette, it is now a separate company under the name Women First Healthcare.
Eflornithine hydrochloride inhibits an enzyme which has been observed to affect hair growth in rats . Regulation of this enzyme, called ornithine decarboxylase (ODC), was later shown to reduce hair growth in mice  and sheep . Recent studies suggest observations in certain types of mice may have parallels in humans. [6, 7]
This drug has been found to be an astonishingly effective cure for some types of African "sleeping sickness," even able to cause a rapid and complete recovery in comatose patients.  Clinical reports show that taking it orally to treat sleeping sickness can induce hair loss as a side effect. 
One large published study on safety found the product rarely caused significant side effects such as acne, follicle irritation, itching or dryness. . This corroborates unpublished data submitted to FDA showing about 2% of subjects discontinued use due to adverse reactions. 
Unpublished efficacy data submitted to FDA observed about 58% of women using it on facial hair had improvement.  This study suggests it may be particularly effective in postmenopausal women.
See my Vaniqa clinical data pages for more on effectiveness and side effects.
Proof of how hard it is to judge effectiveness
Perhaps the most striking result was how many women in the control group (who used cream with no active ingredient) were observed to have less hair. Of 201 patients, over one-third who used a placebo were assessed by physicians as either “improved” or “marked improvement.” 
This huge number of “false positives” means two things for consumers seeking hair removal:
It's really hard to tell if a new hair removal product is effective or not, especially based on the personal experiences of just a few people.
It's really easy for quacks to exploit this difficulty and make overblown claims about products they promote.
That's why you should rely on large-scale controlled studies to determine if a hair removal product is effective.
A note on internet marketing of Vaniqa
Consumers should be cautious when looking to purchase Vaniqa online. Many websites offer come-ons like this:
Free Online Medical Consultation!
No Prior Prescription Needed!
Because it is a prescription drug, and because it's not right for everyone, you should visit a doctor in person before getting a prescription. In fact, it is illegal to sell Vaniqa without a prescription, so many online companies are breaking the law or bending it with a "rent-a-doctor" who rubber-stamps all purchases. This is unsafe and probably against the law. Some consumers ordering prescriptions online have their orders confiscated by postal inspectors or simply never sent, so you run a risk by ordering through fly-by-night companies online.
The official Vaniqa website has a page on how to get it through legitimate means.
Thatte U, Bagadey S, Dahanukar S. Modulation of programmed cell death by medicinal plants. Cell Mol Biol (Noisy-le-grand) 2000 Feb;46(1):199-214.
Griffin CA and others. Phase I trial and pharmacokinetic study of intravenous and oral alpha-difluoromethylornithine. Invest New Drugs 1987;5(2):177-86.
Probst E, Krebs A. Ornithine decarboxylase activity in relation to DNA synthesis in mouse interfollicular epidermis and hair follicles. Biochim Biophys Acta 1975 Oct 1;407(2):147-57.
Soler AP, Gilliard G, Megosh LC, O'Brien TG. Modulation of murine hair follicle function by alterations in ornithine decarboxylase activity. J Invest Dermatol 1996 May;106(5):1108-13.
Hynd PI, Nancarrow MJ. Inhibition of polyamine synthesis alters hair follicle function and fiber composition. J Invest Dermatol 1996 Feb;106(2):249-53.
Nancarrow MJ, Nesci A, Hynd PI, Powell BC. Dynamic expression of ornithine decarboxylase in hair growth. Mech Dev 1999 Jun;84(1-2):161-4.
Panteleyev AA, Christiano AM, O'Brien TG, Sundberg JP. Ornithine decarboxylase transgenic mice as a model for human atrichia with papular lesions. Exp Dermatol 2000 Apr;9(2):146-51.
McNeil, DG Jr. Profits on Cosmetic Save a Cure for Sleeping Sickness. New York Times, February 9, 2001
Pepin J, Milord F, Guern C, Schechter PJ. Difluoromethylornithine for arseno-resistant Trypanosoma brucei gambiense sleeping sickness. Lancet. 1987 Dec 19;2(8573):1431-3.
Hickman JG, Huber F, Palmisano M. Human dermal safety studies with eflornithine HCl 13.9% cream (Vaniqa), a novel treatment for excessive facial hair. Curr Med Res Opin. 2001;16(4):235-44.
Vaniqa package insert (requires Adobe Acrobat). VaniqaTM is a trademark of Bristol-Myers Squibb Company.