This is what I currently keep in my first aid kit:
Oxbow Critical Care
A few jars of plain organic baby food
Benebac - probiotic
Variety of syringes (1cc, 5cc, 10cc, 15cc, 20cc)
A small bowl and spoon for making up Critical Care
Infacol - infant gas relief drops to help with bloat
Betadine
Antibacterial cream
Puppy/kitten Revolution - to treat mites/ringworm etc.
Saline eyedrops
Cotton tips – for cleaning ears and anal glands
Makeup remover rounds – for wiping grease glands, eyes etc.
Sterile gauze swabs – for cleaning wounds
Mineral oil – for cleaning grease and anal glands
Digital scale
Notebook to record weights
Nail clippers
Styptic powder – in case I cut through the quick when clipping nails
Clean white towels – used to wrap uncooperative piggies
Puppy pads – to use in the hospital cage under the fleece/towels
Latex gloves
2x3 "hospital cage" - to use if one of the piggies gets sick and needs to be isolated. I use light coloured fleece and towels inside it as it’s easier to identify blood/discharge.
I should probably also get some gauze and vetwrap for bandaging wounds, but I find most wounds are hard to wrap on a guinea pig so I tend to just keep them uncovered but check and clean them frequently
