First, let me say that PRK is extremely safe.
The most common complication is probably corneal haze after the surgery. This is uncommon and can be treated with steroid drops, superficial excision or more laser treatment. It will actually usually resolve with no treatment if it is left alone long enough but this can take a year or more so most surgeons will treat it. The next most common problem is a possible infection. This typically occurs within the first week as the surface of the cornea is healing. This is generally treated with antibiotic drops which you every patient takes as a preventative measure and again, the likelihood of this is extremely low.
The very worst, but very uncommon complication is corneal ectasia. This is the development of keratoconus after PRK or LASIK. It is not known for certain if the PRK/LASIK actually causes the keratoconus or whether it is simply that people with very early keratoconus. Ectasia patients can not have any further refractive surgery and are put back into glasses if possible. If not, contact lenses are prescribed. If even contacts are not good enough, they may require collagen cross linking therapy or even a corneal transplant. Again, this is very rare but not impossible. Recently studies have shown that patients who develop ectasia were borderline keratoconus patients BEFORE having either PRK or LASIK.
PRK is one of the safest surgeries in existence, but no surgery is totally devoid of potential complications. Remember, your cornea is the fastest healing area of the body.