Medics and state troopers responded to the scene, on I-5 near Anderson Road in Mount Vernon, at about 6 a.m. after receiving a report of a serious crash.
The drivers of both cars were injured. There were no passengers.
A preliminary investigation found that the 78-year-old woman had been heading south on I-5 when for some reason she decided to turn around and head north in the southbound lanes.
She then slammed head-on into a southbound car driven by a 29-year-old Mount Vernon woman.
The 78-year-old suffered chest injuries and possibly a broken wrist. The 29-year-old sustained a broken ankle. Both were taken to a local hospital.
A State Patrol drug recognition expert examined the 78-year-old woman and determined that she was under the influence of some type of prescription medication, troopers reported.
The State Patrol is pursuing the case as a possible vehicular assault.