How allergenic is centella, really?

This study tested the sensitising potential of raw centella extract and of asiaticoside, asiatic acid and madecassic acid individually, using the standard guinea pig model. The extract and all three acids came out as very weak sensitisers. The author's conclusion, quoted directly, was that although centella is frequently applied to damaged skin, the risk of acquiring contact sensitivity to it is low. That is the fair headline, and it is reassuring. What it sits alongside is a small collection of published human case reports going back to 1985 — around ten patients in total, almost all of whom were using medicinal-strength preparations on wounds, scars or ulcers rather than a cosmetic on intact skin. There is no figure for how common centella allergy is in the general population, because no one has ever patch tested a consecutive series of dermatitis patients for it.