We are talking about Viagra, the best-selling drug for erectile dysfunction.

Pfizer, the manufacturer of Viagra, has taken the unusual step of selling its erectile dysfunction drug directly to consumers on its website in an effort to establish a presence in the huge online market. The “little blue pill” is one of the most counterfeited drugs in the world.

We are not talking about chump change here. Pfizer admits that Viagra alone brought in more than $2 billion in sales in 2012. Many experts estimate that Pfizer could be losing hundreds of millions of dollars a year to a huge black market of online pharmacies that cater to men too embarrassed to buy the drug through traditional sources.

Pfizer has made a deal with CVS/pharmacy that allows patients in the United States with a valid prescription for Viagra to fill their order through the new website, which prominently features the slogan “Buy real Viagra.” Patients will need a valid prescription from a doctor but won’t need to visit the pharmacy.

The online sale of Viagra is attractive because the drug has powerful brand recognition. There is widespread competition from counterfeiters, and Pfizer hopes to capitalize on this market.

According to the research firm IMS Health, about eight million Viagra prescriptions were written in the United States last year. Victor Clavelli, a marketing executive at Pfizer, reports that Viagra appears in about 24 million Internet searches a year, often in phrases like “buy Viagra.” There is clearly a disconnect. “Our goal is to just make sure those patients actually get the real Viagra,” said Mr. Clavelli.

Arriving on the market in 1998, Viagra was the first oral medication specifically indicated for erectile dysfunction. Since that time, Pfizer has gone to great lengths to make erectile dysfunction mainstream and to take the stigma away from male impotence. The “little blue pill” has become enormously popular and is widely used. But because of the drug’s popularity and its relative high cost, a vigorous black market developed and expanded as online commerce flourished.

Matthew J. Bassiur, vice president of Pfizer Global Security, said that the company had seen counterfeit medicines manufactured “in filthy and deplorable conditions.” He also noted that some samples of counterfeit Viagra tested by Pfizer had contained “pesticides, wallboard, commercial paint and printer ink.”

It is clear that many people do not realize the terrible risk that taking counterfeit Viagra poses to their general health. These results “motivate us to continue our aggressive global efforts to stop those who prey on unsuspecting patients,” said Mr. Bassiur.

In 2011, Pfizer evaluated 22 websites appearing in the top search results for the phrase “buy Viagra.” Chemical analysis found that about 80 percent of the pills were counterfeit. In addition, the fake Viagra pills contained only about half of the active ingredient (sildenafil citrate) compared with the legitimate product.

But many pharmacies, based both in the United States and abroad, sell legitimate drugs online. These medicines require a doctor’s prescription. Purchasing drugs online from a foreign country is technically illegal, but the federal government does not generally prosecute individuals who purchase medicine in small quantities for their own use.

A spokesman from Pfizer notes that it is difficult to distinguish the legitimate pharmacies from the illicit ones, but the company hopes that the Pfizer brand recognition will reduce the risk of buying a counterfeit.

The cost of the drug remains a problem. The average list price for Viagra is about $22 a pill, while many online pharmacies sell the pills for about $10. Price could become an important factor, but Pfizer hopes the decreased risk of purchasing a bogus and possibly harmful pill will offset the higher cost.

The good news is that customers who buy Viagra through the Pfizer website receive three free pills in their first prescription (which usually contains six pills) and 30 percent off their second prescription. This same promotion and discount, however, is available in a doctor’s office as well. Many in the pharmaceutical industry see Pfizer’s move as part of a continuing effort to market drugs directly to consumers and bypass insurance companies that can be reluctant to pay for so-called lifestyle drugs.

Viagra is the dominant drug for sexual dysfunction (taking 49 percent of the market, followed by Cialis at 39.7 percent and Levitra at 8.6 percent) and is now available online. The bottom line? Be careful what you buy on the Internet—and be sure that the product you receive is the product you paid for.

Dudley S. Danoff, MD, FACS is the attending urologic surgeon and founder/president of the Cedars-Sinai Medical Center Tower Urology Group in Los Angeles, California. He is the author of Penis Power: The Ultimate Guide To Male Sexual Health (Del Monaco Press, 2011) and Superpotency (Warner Books).

Read discreetly with the Kindle™ edition of Penis Power™ now available for purchase from Amazon. The Nook Books™ edition from Barnes & Noble and the Sony eReader™ edition from Sony’s Reader Store. Available for under $7.00!