Sophie O'Hagan was last seen leaving her home address in Grangemouth at around 6.20pm on December 31, but has not been seen or heard from since.
Ms O'Hagan is described as being around 5ft 5ins and of slim build with long auburn coloured hair.
Police have stated that the pink colour of hair that Ms O'Hagan has in the photo they have distributed has since been changed.
At the time she went missing, the teenager is said to have been wearing a black jumper, blue jeans and grey trainers.
Ms O'Hagan is thought to be using trains to travel, as she is said to do so regularly, as well as having known links to the Glasgow and Lanarkshire areas.
Inspector Andrea Campbell said: "Sophie has now been missing for a number of days and we are becoming increasingly concerned for her welfare and are keen to locate her as soon as possible.
"We have a number of police resources out looking for Sophie and know she has been in both Cumbernauld and Uddingston since she was reported missing on Tuesday, December 31.
"I would urge anyone who may have seen Sophie to contact us. Similarly, if Sophie is reading this, let us know that you are safe and well."