When a sick ferret requires a bit of fattening up, many people recommend different supplements and foods such as ferretvite (in the tube, not the multivitamin water additives), cream, ensure, baby food and so forth. One thing that I have never heard recommended, and I feel should certainly be used is Whiska's cat milk, rather than Ensure.
Cream is low in sugar, however, due to the lactose, can sometimes irritate the stomach of ferrets with intestinal issues; Ensure, though low on the lactose end, is extremely high on the sugar end and is generally relatively low in animal proteins; Catmilk is low in terms of lactose and though it is higher in sugar than cream, it is a much better alternative than Ensure!
Cat milk is a high fat, reduced lactose cream supplement designed specifically for cats who, like ferrets, are often unable to digest lactose as adults. In my experience, I have found that most ferrets that will drink Ensure LOVE cat milk. Cat milk provides the same benefits that Ensure is supposed to with more benefits. Cat milk can also be used to hide the taste of small amounts of Pepto-Bismol (which is great for preventing ferret tummy aches), and, as an added bonus, cat milk is fortified with a small amount of taurine - something which is definitely not in Ensure!
*** Cat milk has a higher sugar content than cream and is not recommended for ferrets with insulinoma; it is best used for recovery of extremely sick or palliative ferrets and avoiding for young, healthy ferrets unless the situation is one in which the ferret needs to gain weight quickly or recover from a surgery and he or she cannot or will not eat solids or dook soup as there are concerns that sugars such as malt can contribute to the development of insulinoma later in a ferret's life. Cat milk is a much safer and lower sugar alternative to Ensure and other human - oriented dietary supplements.