Does retinol cream firm your skin? Or does AHA? Does it help with your neck after losing weight?

Retinol creams, especially those that you can buy without a prescription, will not firm your skin. Tretinoin, or Retin-A, however, when applied regularly will create new collagen in the support layer of the skin which can, with time, tighten it. When Obagi's Elastoderm, which converts the collagen to elastic tissue, is added twice a day, the process is speeded. AHA's which reduce the build up of dead skin cells play no role in tightening the skin other than making it easier to absorb the Retin-A and elastoderm, but are an important part of normal skin care along with a good UVA sun block like Zinc Oxide.

There is no effective topical cream that will tighten the skin very much, especially on the neck after weight loss. You should consult with a plastic surgeon or cosmetic dermatologist for options, but no creams can do what you want, it's just not possible. "This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."