Hadacol was a patent medicine popular in the United States in the 1940s. Hadacol was developed and marketed by Dudley LeBlanc, a Louisiana entrepreneur and state senator. LeBlanc claimed that he stumbled upon the recipe for Hadacol when his own doctor administered a seemingly miraculous treatment that, LeBlanc claimed, brought LeBlanc back from the point of death. LeBlanc made outrageous claims for Hadacol, marketing it as a cure for virtually every illness known to modern medicine, including cancer, diabetes, arthritis, epilepsy, and hay fever. Hadacol enjoyed enormous popularity on the U.S. market, even though it consisted mostly of alcohol and vitamins and had little medicinal value.

Hadacol is considered by many to have been the last patent medicine. Many believe that LeBlanc's marketing techniques drew strongly upon the 19th century American tradition of the traveling medicine show, in which salesmen pitched often useless, and sometimes even harmful, home remedies by way of vaudevillian entertainment.

Hadacol contained about 12 percent alcohol, as well as B vitamins and minerals such as iron, calcium and phosphorous. Hadacol also contained dilute hydrochloric acid, which, LeBlanc claimed, allowed the formula to absorb more easily into the body. A small bottle of Hadacol normally sold for $1.25 US Dollars (USD), while the larger size bottle typically fetched $3.50 USD. LeBlanc successfully marketed this patent medicine through a multi-pronged campaign that included old-time traveling medicine shows, traditional advertising, comic books, and jingles.

According to LeBlanc's claims, both adults and children could benefit greatly from Hadacol. Adults and children over 12 were advised to mix one spoonful of Hadacol with half a glass of water four times per day. Children aged 6 to 12 were advised to take three spoonfuls of Hadacol four times per day, while children aged two to six were advised to take two spoonfuls of Hadacol four times per day. LeBlanc claimed that Hadacol could cure almost any illness, including gout, asthma, pneumonia and high blood pressure.

While those who used Hadacol often reported that it relieved their symptoms, the United States Food and Drug Administration (FDA) found no evidence to support LeBlanc's claims, and he was obliged to withdraw them. The United States Federal Bureau of Investigation (FBI) found that the sole physician who endorsed Hadacol, a Dr. L.A. Willey, in fact carried a previous conviction for practicing medicine without a license. In the end, LeBlanc sold the Hadacol company to the Tobey-Maltz Foundation of New York, an organization devoted to cancer research. The foundation, however, quickly discovered that the Hadacol company was deeply in debt, and that they, not LeBlanc, were responsible for paying off the creditors.