Any illness in ferrets can quickly become lethal if not treated quickly enough. But ferrets can be quite clever at masking signs of pain. This was probably a survival adaptation. Any animal that looked sick quickly got the attention of any local predators. But there are ways to determine if your ferret is in pain.
Changes In Appetite
This is a big giveaway. Because ferrets have such a s short digestive system and a high metabolism, they need to eat every few hours. If a ferret suddenly loses its appetite, something is definitely wrong. If a ferret only takes a couple of bites and leaves most of the food alone when it normally acts like a food vacuum cleaner, then that can also be a sign of pain.
Ferrets can have problems with their teeth or jaws, which usually manifests as the ferret having problems eating, even though it is really hungry. The ferret may only eat soft foods or tilt the head to one side while eating or drop the food after unsuccessfully trying to chew it. The ferret may also stare at the food and sniff it but not eat it.
Changes In Allowing Touch
Ferrets are quite vain. Even though they seem to not care about where they poop, they usually are quite fastidious about grooming themselves. If they stop grooming themselves, this is usually a sign of pain.
Or perhaps the ferret is over-grooming one area of its body. If it keeps licking one part of the body, it could be because that part is in pain. The ferret may also suddenly become protective of that part of the body, not letting human nor ferret get anywhere near it. If anyone does manage to succeed in touching the area, they may get hissed or snapped at. The ferret is suddenly aggressive due to panic set in by pain.
Although ferrets enjoy burrowing, they do come out of their hidey-holes at predictable times or if food is on the offer. But if the ferret is still hiding when it normally is out and about, this is a sign to check it further. If the ferret is squinting, there could be pain in the head or eyes and normal light may hurt it. The ferret's eyes should be glossy. If not, it could be in pain due to eye problems.
If the ferret is suddenly quiet and refused to play and sits in a corner with the tail puffed out, then it could be in pain. Any strange body posture may mean the ferret trying somehow to get comfortable. The ferret may also tremble, quiver or make strange noises at times it is usually quiet.
No OTC Drugs
It may be tempting to give your ferret a dose of painkiller that's worked for you. Don't do that. Over the counter painkillers like acetaminophen (Tylenol), ibuprofen and naxoproxen are poisonous to ferrets. Even a fraction of a pill can make a ferret's internal organs begin to shut down.
Instead, get the ferret into a dark, warm and quiet nook, away from any other rambunctious pets or household members and call the veterinarian.