It is just as well that the royal guard does not examine Dr. Margery G. Blackie's bag when the queen's physician makes house calls at Buckingham Palace. If they did, they would find arsenic, strychnine, wormwood, wolfsbane, death cap mushroom, and the venom of the Gila monster, rattlesnake and hooded cobra. It may sound like medieval sorcery, but these are the working tools of homeopathy, a branch of medicine represented among the royal physicians since the early 19th century.

"In homeopathy," Blackie explains, "we believe the thing that causes the disease can also cure it. I follow the principle of treating like with like." Hangover sufferers know the concept as "hair of the dog." Standard medicine practices it in the form of vaccination.

Homeopaths—who are almost always also M.D.s—treat hay fever sufferers with grass cuttings and pollen, for instance. The cures are given in low dosages and mixed with sugar, alcohol and lactose as dilutants. Homeopaths point to increasingly drug-resistant bacteria and the side effects of synthetic drugs as arguments for their all-natural treatments. "Less medicine," Blackie argues, "is better."

In the United States, homeopathy enjoyed brief popularity among doctors in the 1840s just after the death of its founder, German physician Dr. Samuel Hahnemann. It has since fallen into disfavor—and sometimes scorn. Only a few hundred doctors in the U.S. use homeopathic techniques today.

But homeopaths have been welcome in Britain's royal household since Queen Victoria's "Uncle Leopold" (later King Leopold I of Belgium) consulted one in 1827. Blackie was appointed to her post in 1969; she was the first—and so far only—woman physician to the crown. Its medical staff includes three other doctors, with a half-dozen specialists on call. Blackie's relationship with Elizabeth II is cloaked in discretion. The doctor will say only: "The Queen is a wonderful person—and very healthy—with a good sense of humor." Among homeopathic treatments, however, Blackie is thought to give the Queen special flu pills in the winter. Her Majesty is seldom ill, court observers note.

Blackie ministers to nonroyal patients in the fashionable Kensington townhouse she shares with her friend and secretary of 35 years, Lucette Majendie. She also serves as dean of faculty at the Royal London Homeopathic Hospital, which listed King George VI as a patron during his reign.

Her homeopathic treatments have worked best, Blackie says, against migraine, shingles, respiratory diseases and rheumatism. Her prescriptions range from snake venom for shingles to poison ivy and strychnine for rheumatics. She dispenses mercury for diphtheria, sulfur for eczema and club moss for insomnia. She has even treated cancer with mistletoe—successfully, she claims.

Blackie often gets referrals from nonhomeopathic doctors and reciprocates when homeopathic techniques can't help. "There are still gallstones and appendixes," she acknowledges.

The youngest of 10 children in an intellectual Scottish family, Blackie was inspired by her maternal uncle, James Compton-Burnett, father of novelist Ivy and a well-known homeopath. "I wanted to be a physician from the age of 5," she remembers. She grew up in Hertfordshire and Surrey and qualified as an M.D. in 1928.

On long weekends she and Majendie retreat to Hedingham Castle, a 100-acre estate which has been in Majendie's family for more than 200 years. Blackie gardens, bird-watches, plays croquet and prepares lectures. She also dispenses homeopathic treatment to the neighbors, human and otherwise. She is especially proud of having cured one old friend, a 300-year-old tulip tree that was dying of what seemed like acute anemia until she dosed it with bonemeal and dried cattle blood.