Even if you think you have got your head around the differences between alpha hydroxy acids (AHAs) and beta hydroxy acids (BHAs), not to mention the subsets thereof like glycolic acid, it can still be difficult to choose the right product and regime. Especially when one's skin has a split personality. Most of us can relate to the member of the Truth In Aging community who wrote in and said: "I am 53 and have combination skin (which tends to be very oily in summer and rather dry in winter) and am STILL prone to break-outs." Should she use AHAs or BHAs? Conceivably both, but I am running ahead of myself.

First a quick reprise. Alpha hydroxy acids are clinically proven to loosen the glue-like substances that hold skin cells together, thus shedding the top layer of dull, damaged skin. Glycolic acid is a type of alpha hydroxy acid (AHA) naturally found in sugar cane. As the AHA with the smallest molecule and the greatest penetration, glycolic acid deeply exfoliates to remove skin that is dry, coarse, or dead. Ammonium glycolate is a form of glycolic acid. Other AHAs include mandelic acid, made from bitter almonds, citric acid, malic acid (from apples) and lactic acid.

Unlike alpha hydroxy acids, which are water soluble, beta hydroxy acids are lipid (oil) soluble. This means that they not only exfoliate the upper layer of skin, but also penetrate deep through the epidermis to exfoliate the dead skin cells and excess oil built up in the pores. For this reason, BHA’s are frequently used to treat blackheads, whiteheads and acne. Yet its keratolytic ability is not the only reason for why it is so often administered to treat blemishes and breakouts. Beta hydroxy acid also functions as both an antimicrobial and anti-inflammatory agent, making it an ideal anti-acne ingredient. BHA is organic acid that’s either biosynthesized or extracted from the bark of a willow tree. The compound is known as salicylic acid or salix alba extract.

So what to use when? This is a largely personal view, but here goes. I am a great believe in horses for courses. Every morning, I peer into the mirror and try to access what kind of a skin day I am about to have. If I look prone to a breakout then I will reach for a cleanser such as Epicuren's or PrescribedSolutions, which has salicylic acid.

However, I wouldn't use BHA/salicylic continuously, or at all if I was pregnant. Reproductive and developmental toxicity have been associated with exposures to large, therapeutic serum concentrations of Salicylic Acid (as a metabolite of aspirin). That means you'd have to be exposed to an awful lot, but even so.

Back to AHAs then. How much and how often rather depends on your sensitivity levels and the state of your skin. If your skin is rough and dull, then a potion with glycolic (see our Five Best for inspiration) could be used four or five times a week. I would definitely use a collagen boosting serum over the top. As the skin clears, you can start to ease back. I first started using IMAGE Skincare every other day a year later I am down to a couple of times a week on those days when the Clarisonic Brush isn't quite enough.

High concentrations of glycolic acid, such as the 29.5% in Glytone should be reserved for heels and elbows. Because this top layer of skin provides some (albeit minimal) protection from the sun, there is an increased risk of sun sensitivity after using an AHA.