One of my favorite researchers, Dr. Zoe Draelos, has published an article on Skin Lightening products in Skin Inc. magazine (A magazine for skin care professionals).
The main points from the article
All OTC skin-lightening treatments should include using a sunscreen daily.
Hydroquinone remains the gold standard in skin lightening, but OTC products may be banned by the FDA.
Mequinol a prescritpion based product that does not damage the melanocyte like hydroquionone does.
RETINOIDS are typically combined with other skin lightening agents to enhance penetration. Retioids do an effective job of lightening freckles. OTC retinol has been thought ot provide similar effects that prescription strength retinoids.
Vitamin C when used by itself is a poor skin lightener.
Licorice extract is low on the irritation scale, but must be used in high concentrations over a long period of time.
Alpha Lipoic Acid' s ability to lighten skin is doubtful
Kojic acid is effective but has a hard time penetrating the skin and needs to be used with penetration enhancers like a retinoid.
Aleosin & Arbutin are not very effective but need to be mixed together to be effective.