Is is safe to have lip filler before going into hospital for a surgery under general anaesthetic?

Go right ahead!  There is no problems or contraindications for you to have filler prior to general anesthesia.  Good Luck with everything

There is likely no risk to you if this is done, especially if done with a cannula, however, i suggest waiting. there is likely no reason why you would need to do this prior to having general anesthetic which would need access to your throat.  I suggest waiting till after.  Best, Dr. Emer.

I would generally advise against any elective cosmetic procedure just prior to a medically necessary procedure to avoid any delay or interruption of medical care.