Marina Kovalevskaya, 49, and her daughter, Yana Kovalevskaya, 26, who emigrated to the United States in the 1980s, were hospitalized February 24 in Moscow with symptoms that the service overseeing consumer protection and welfare said earlier Tuesday might have been caused by thallium, a chemical used in rat poison and insecticides.
Thallium is odorless, tasteless, and can be easily dissolved in water.
Moscow's chief health official, Nikolai Filatov, said Tuesday, "The consequences of such poisoning depend on the individual, the dose and the duration of contact [with the toxic substance]."
The U.S. Embassy has not confirmed that the women were poisoned with thallium, but said it was a possibility.
In the case of former Russian intelligence officer Alexander Litvinenko, who died last year in London, doctors also initially suspected thallium poisoning. A closer medical examination subsequently revealed a large dose of radioactive polonium-210 in his body.
Investigators are looking into the reasons and circumstances of the women's poisoning.