A serum you use once a night lives somewhere for about three months. Where that somewhere is turns out to matter, and the reasons are mostly unglamorous.
Upright, always
A brush applicator wipes clean against the neck of the bottle on the way out. Stored on its side, product settles against the cap and the wipe does not work, so you carry more onto the lash line than you meant to — and the bottle empties early.
Out of the bathroom window
Direct sunlight and heat are what age a formulation. A windowsill above a radiator manages both at once. A drawer, a cabinet, or anywhere on a bedside table that is not in the sun is fine. There is no need to refrigerate it, and a cold bottle is less pleasant to apply.
Cap closed, properly
Not because anything dramatic happens if you forget once, but because an open bottle lets the liquid concentrate as it evaporates and lets the brush dry out. A brush that has dried out drags on the lash line instead of gliding.
One person, one bottle
The applicator goes back inside after every use, which makes the bottle a closed loop between your lash line and the liquid. That is fine as a loop of one. Shared between two people it is not a loop anyone should want. The same logic applies to putting a dropped applicator back in: replace the bottle instead.
How long an opened bottle lasts
Check the small open-jar symbol on the carton — the number in it is the number of months the product is intended to be used for after opening. At one application a night, a 3 ml bottle is finished well inside that window, which is the tidiest way to manage it: open one, use it nightly, finish it.
And if it changes
If the colour, the smell or the texture shifts noticeably, stop using it. That is true of any cosmetic and is not a sign you did something wrong. Write to us and we will sort it out.