Why did the wrinkles on my upper lip go away at the 3 week mark of CO2 laser resurfacing and reappear at 6 weeks?

Commonly, the re-appearance could be from the resolution of swelling (edema). However, with a good CO2 laser treatment you will continue to "build collagen" beyond this time point and may see more improvement in the wrinkle.The attached link provides more information.I hope this helps.

Swelling is common with laser treatments and can vary in length of time it is present depending on type and extent of treatment.  Sounds to me like some of the initial swelling was obscuring the lines which gave the appearance that they had resolved.  Once the swelling began to resolve, the lip lines became apparent again but hopefully better then they were at baseline.  Regardless, collagen remodeling continues to occur up to 3 months after your treatment so I would give it time.  If you have concerns, discuss this with the doctor who performed your procedure to alleviate some of your uncertainties.  Best of luck!

It's too soon to tell what your final results will be. It can take 6-12 weeks for laser results to settle. I recommend following up with a cosmetic dermatologist for after care and skin care recommendations.Sun protection is essential. Lasers like Co2 work best in combination with other treatments in order to improve the skin's texture and collagen. After Co2, I recommend adding fraxel lasers, microneedling/prp and radiofrequency treatments. See an expert. Best, Dr. Emer