You start a promising new active, and within a couple of weeks your skin breaks out. Is this the fabled "purge" — a temporary flare as the product speeds things up before improving — or is the product simply wrong for you? Getting this judgment right stops you from both abandoning something that would have worked and persisting with something that is harming you.
There are some useful clues to tell the two apart.
What purging is
Purging is associated with ingredients that increase skin cell turnover, which can bring already-forming congestion to the surface faster than usual. If it happens, it typically appears in the areas where you normally break out, tends to resolve over a few weeks as skin adjusts, and then gives way to improvement.
Crucially, purging is only relevant to certain kinds of actives — it is not an all-purpose excuse for any product that makes your skin worse.
What a reaction looks like
If breakouts appear in new places you do not usually get them, or if the skin is red, itchy, stinging, swollen or clearly irritated rather than just spotty, that points toward a reaction or an unsuitable product rather than purging. A product that does not belong to the turnover-boosting category, causing new congestion, is also unlikely to be "purging" — it may simply be clogging you.
Irritation and reaction are signals to stop; purging is a signal to wait a little longer.
How to respond
If it looks like purging — right ingredient, usual areas, settling over weeks — it is reasonable to continue gently and give it time, perhaps easing the frequency. If it looks like a reaction — new areas, irritation, worsening — stop the product and let your skin recover. When unsure, erring toward caution and simplifying the routine is the safer default.
And introducing one new product at a time makes this whole judgment far easier, because you actually know what to blame.