They say police received a call about the crash on the highway, about 50 metres east of the Highway 91 junction, just after 4:15 a.m. Sunday.
The RCMP say a 68-year-old woman driving a Ford sedan struck a Mercedes sedan head-on.
They say both vehicles were significantly damaged, and the driver and passenger of the Mercedes were also brought to a hospital with minor injuries.
They say investigators believe alcohol and a "pre-existing medical condition" are factors in the crash.
Police say they closed the southeast lanes of Highway 99 to investigate, and are asking anyone with information or dashcam footage of the incident to come forward.
