An investigation began in late November when an 84-year-old woman at the home near Chambery went into a coma suddenly and died two days later, Franceinfo.fr reported. An autopsy found indications the resident had been poisoned.
The suspect was arrested Wednesday.
Five other deaths in the previous two months are considered suspicious.
The suspect, described as a woman in her 30s, allegedly told investigators she "wanted to alleviate their suffering."