Can you tell us what medicine they gave you? I’ll link to syringe feeding guide below.
Complete Syringe Feeding Guide
You could also feed him off a spoon. If using the syringe just don’t let the other piggy get any. You may have to take the piggy out the cage for it.
I don't know exactly what they gave me, which is common in human medicine here as well - they give you packets and tell you to take twice a day, and that's mostly it (humans can check the script they take to the pharmacy, if they want, but I was in a tizzy and we were at the vet for what felt like quite a while, until their closing hours, so I didn't think to ask exactly what we were getting). I know that one med is an antibiotic, and the other is a probiotic (probiotic taken first, antibiotic 15 minutes later). Not sure about the humidifier liquid (all three literally say "probiotic, internal medicine, nebulizer solution" when translated).
I got (I'm pretty sure most) of both meds in him this morning, using the techniques from that guide. He ate the probiotic himself, but I had to hold him for the antibiotic (which the vet had warned me was likely to be necessary, as it doesn't taste good). I gave him a couple bites of carrot after each, to push down the meds and reward him (probably won't use carrot every time, but I'll certainly give him something after). I did have to put a quick cage separator up when syringe-feeding, because his cagemate is very active and likely would have gotten in the way (he pulled very hard on the separator, but couldn't get past it - it was 2 grids with a single grid attached on the side for stability. I'd also like to note that I did give him food to distract him, but that didn't hold him the whole time.).
The piggies were fine once reunited after the med feeding, and the sick one seems to be active and eating and drinking normally, as he was before.
