I received a large jar of Retin-A from my doctor 11 months ago and I still have quite a bit left b/c I haven't used it consistently. My brown spots and old scares are worse then ever so I plan to commit to using the product as he directed almost a year ago. Is it safe and still effective if the product is almost a year old? Does Retin-A expire? If yes, how long will a jar last in the medicine cabinet?