The family of 52-year-old Alvin Lamont Breeden is also pleading for help.
Police told ABC11 he was last seen on March 1 at a relative's home on Hemlock Avenue.
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Breeden's sister Kathy is concerned because her brother hasn't touched his bank account. Also, he had a doctor appointment recently that he didn't cancel.
She said he battles PTSD and takes medication but he's been handling it as of late.
"He was a real people person, he wouldn't hurt anyone, so we just want to know what happened and where he is," she said. "I'm his oldest sister and I talk to him every other day on the phone if not sometimes every day. He would've never disappeared without calling my mom to check on her."
"But for him to just disappear from a whole month," she said, "no, there's no way."
Breeden is described as a black male, 5' 10", with a medium build. He was last seen wearing a red and white shirt with short sleeves, blue jeans, a black jacket with red trim, and red and orange sneakers.
Investigators said he's known to frequent the areas of Fayetteville and Pilot streets as well as Crimson Creek Drive, Hemlock Avenue, and Woodcraft Parkway/Barbee Road.
Anyone with information on his whereabouts is asked to call the Durham Police Department at (919) 560-4427 or 911.