Glycolic acid is stronger and works faster but can over-strip combination skin, while mandelic acid is gentler and better tolerated if you're oily in the T-zone but dry everywhere else.

• Glycolic acid has the smallest AHA molecule, so it penetrates fast and delivers quick results – but that speed can mean more irritation.
• Mandelic acid's larger molecule penetrates slower, giving you resurfacing without the same risk of over-exfoliating drier areas.
• Mandelic is also naturally antibacterial, which is a bonus if you deal with breakouts in your T-zone.
• If you're new to chemical exfoliants, start with mandelic and build tolerance before trying anything stronger.
• Both need consistent SPF, since AHAs increase sun sensitivity.

2-3 times a week is the max with most AHAs; over-exfoliating shows up as sudden sensitivity, breakouts and tightness, which is really just a damaged barrier – switching to a leave-on mandelic tonic every other night usually solves it.

• Over-exfoliation looks like stinging from products you used to tolerate, unexpected breakouts, and skin that feels tight even after moisturising.
• Most AHAs and BHAs are best capped at 2-3 uses a week, especially when you're starting out.
• A leave-on tonic like mandelic acid, used every other night, gives you results without the sensitivity that comes with daily strong acids.
• If you're already over-exfoliated, pause actives completely and let your barrier recover for a week or two first.
• SPF isn't optional here – it protects skin that's already more vulnerable from exfoliating.