There are three common ways to make a lash line look fuller, and they are usually discussed as if the only difference were price and how long the effect lasts. The more useful difference is what each one asks of the hair.
Mascara: a coating, and a removal problem
Mascara adds nothing permanent. It coats the hair and comes off the same night, which makes it the gentlest of the three in principle. In practice the cost is at the sink: the film is built to resist water, and the harder it resists, the more rubbing it takes to remove. Almost all mascara-related lash loss is mechanical and happens during removal, not during wear.
Waterproof formulas move the whole problem up a level. They are worth keeping for the occasions that need them rather than as a default. How to take it off without paying for it in lashes.
A lift: a chemical change to the hair you already have
A lift softens the bonds inside the keratin, holds the lash against a shaped shield, and sets them again. Nothing is added and nothing is glued on — the hair itself is re-shaped, and keeps that shape until it is shed. The cost is that processed hair is more porous for a while, and that a tint in the same appointment is a second process on the same day. What that means for aftercare.
The interval technicians ask for between appointments is not a sales rhythm. It is the time the lash cycle needs to replace the processed hair.
Extensions: weight, adhesive, and the removal
Extensions add a hair to a hair. Each natural lash carries an extension until it is shed, which is why a set loosens gradually rather than all at once. Two things carry the cost here: weight, where an extension is longer or heavier than the lash it sits on, and removal, whether that is a professional dissolve or the far more expensive version where somebody picks at them.
The week a set comes off is also the week the lash line looks its worst, and that is mostly contrast and timing rather than damage. The timeline, week by week.
If your lashes already feel fragile
Stop in this order. First the curler, particularly a heated one — it costs the most and adds the least. Then waterproof mascara, or at least the daily use of it. Then, if you are between sets anyway, give extensions a cycle off rather than booking again to cover the gap. A lift sits between the two: gentler than extensions, not as neutral as mascara.
None of this argues for doing nothing. It argues for knowing which of the three you are paying for in hair, so the decision is yours rather than the appointment's.
Whatever you choose
Be careful at the lash line when you cleanse, give any change two full months before judging it, and stop anything that stings, reddens or itches. Before the first use of any new product on the eye area, patch test.