What harms a fragrance
Direct sun, temperature swings and humidity alter fragrance oils. The bathroom is therefore a poor place to keep them.
How to tell a changed fragrance
The colour gets darker, the scent smells sour, or the fresh top note is missing. Then it has usually aged noticeably.
Conclusion
Store cool, dark and dry, ideally in the original box and not in the bathroom.
Read next: How to apply perfume · Scent Finder
