Giardia is an elusive target for lab technicians, sometimes completely escaping microscopic examination. An antigen assay test, which would make Giardia detection quicker and more certain, has been developed by scientists at Johns Hopkins in Baltimore and the National Institute of Allergy and Infectious Diseases in Bethesda but is not yet approved for general use.

In the United States, about 4 percent of stools submitted for parasitic examination contain cysts (dormant, encapsulated forms) of the organism Giardia lamblia, which causes giardiasis.

Drugs prescribed for giardiasis are:Atabrine (generic name quinacrine), a drug developed for the treatment of tapeworm and malaria but now widely used for giardiasis. It comes in 100-milligram tablets that adults usually take three times a day for five days. Merck's Manual, a desktop reference for physicians, credits the drug with a 70-95 percent cure rate but warns that it "may produce gastrointestinal disturbances and, rarely, toxic psychosis." Flagyl (metronidazole), originally intended for the treatment of a sexually transmitted disease called symptomatic trichomoniasis and now also approved for a variety of other infections. Giardiasis is not on the FDA approved list, but the drug is widely (and effectively) used for it. Merck's Manual suggests one 250-milligram tablet three times a day for seven days for adults, but some physicians prescribe larger doses. There have been some laboratory animal studies suggesting a cancer link.Furoxone (furazolidone), a broad-spectrum drug for treatment of intestinal diseases, including cholera, shigellosis and staph infections as well as giardiasis. It comes in both tablets of 100 milligrams, recommended four times a day for five days for adults, and a liquid form suitable for children. Furoxone interacts badly with alcohol (like the aversive drug Antabuse), so doctors warn patients starting on the drug not to drink until four days after therapy is over.