Alum is the name generally given to two common salts: potassium aluminum phosphate and ammonium aluminum sulphate. In its naturally occurring state, the substance has been used for more than 2,000 years in a wide variety of applications. Before the beginning of the 20th century, it was routinely manufactured and used for pickling, canning, tanning leather, and baking. From then on, its use in food processing gradually decreased because of safety concerns over its side effects. Professionals in the fields of medicine and health care, however, still frequently use alum to treat injuries and disease.

In medicine, alum is regularly employed as an astringent to shrink tissues and reduce the discharge of bodily fluids; a styptic to contract organic tissues and stop or reduce hemorrhage and bleeding; and an emetic agent to induce vomiting when someone has ingested poison. It is also often used to enhance certain vaccines and to prevent or treat infections. Veterinarians typically advise pet owners to apply the powdered version to animal cuts caused by improper nail trimming as a way to stop bleeding.

When alum is used to enhance a vaccine, it is classified as an adjuvant, or supplementary contributing agent. Adjuvants containing aluminum have been shown to make some vaccines last longer and appear to often help produce more antibodies against disease. Relatively common vaccines licensed in the United States and typically administered to children include diphtheria-tetanus-pertussis (DTP), where pertussis is commonly known as whooping cough; diphtheria-tetanus-acellular pertussis (DTaP), acellular meaning no whole cells of pertussis; Hepatitis A and B; human papillomavirus (HPV); and rabies.

Some people believe that powdered alum is an effective home remedy for canker sores. Advocates of this form of treatment typically claim that dabbing the sore with the powder a few times a day will speed up healing. Anyone adopting this method of treatment should, however, be aware of the possible side effects. Sores may sometimes burn and the powder can cause puckering of the mouth on occasion. It is also recommended for those applying the powder in or near the mouth to rinse their mouths with water following treatment since the emetic effects of alum could cause vomiting.

Alum is still an ingredient of some everyday products found in many medicine cabinets. For example, people often use styptic pencils — short sticks of molded alum with points like pencils — to stop bleeding from razor cuts from shaving. The substance is also frequently listed in the ingredients of toothpaste and tooth powder.