Billy Fair, 22, and Jessica Pagan, 18, left the Redwood Place mental health rehabilitation center at 18949 Redwood Road on Wednesday at around 10 a.m.
They left the facility on a 30-minute pass and never returned, according to sheriff's Sgt. Kyle Ritter.
Fair and Pagan are both developmentally disabled and take prescription medications. They both left the facility without their medications and sheriff's officials said Fair in particular is likely to suffer adverse effects without his seizure medication.
Family members and staff said that Fair and Pagan have never been away from Redwood Place for so long and believe the two may be headed to San Francisco or Union City, Ritter said.
Fair was described as a white male with brown hair and blue eyes. He is 6 feet tall and weighs 186 pounds. He was last seen wearing a striped, cream-colored polo shirt, khaki shorts and rubber shoes. In a picture given to the sheriff's office, Fair was wearing glasses.
Pagan is a black woman, 5 feet tall and weighing 187 pounds. She was last seen wearing cream-colored pajamas and black shoes.
Anyone with information regarding their whereabouts is asked to call Sgt. Dave Dickson at (510) 667-3921.