Glycolic acid and retinol isn't the best combination in a skincare routine because retinol is a component that increases cellular turnover and sends young cells toward the skin's surface, while glycolic acid will continue to remove them, which will most likely lead to irritation. Therefore, if you're already using one but want to try the other, make sure to give your skin a long break between stopping an active ingredient and introducing a new one so that it can adjust accordingly without becoming irritated.