The recommended guideline is for fillers to be used in a single session in view of the risk of skin contamination.
There have been previous studies which reported no contamination of partially used hyaluronic acid fillers.
However, this particular study stands out. M Al-Haddab et al studied the potential of infectious contamination of hyaluronic acid fillers kept in storage (32.87–71.57 months after initial treatment). The study was published in April 2017 and reported that although no fungal or mycobacterial agents were cultured, 5 of 36 syringes were contaminated with bacteria commonly found on normal skin. He concluded that the usage of stored, partially used fillers carries a small but real risk of contamination.
Personally, I review patients 2 weeks after filler injections and would prefer to use any remaining filler at this point to minimise the risk of contamination.
I hope this helps!
Dr Edwin Lim