Ferretone and Linatone are two popular vitamin supplements.  They are
also one of the most common treats, since nearly every ferret loves
them.  They're very similar and can be used interchangeably, although
their exact composition is a bit different.  Both of these contain
vitamin A, which can be very harmful or even fatal in excess, though

it probably takes a whole lot more than you'd ever give your ferret.
Still, some people prefer to dilute them 50/50 with olive oil or
vegetable oil (<NOT mineral oil), which shouldn't hurt.  Also, as with
hairball remedies, too much Ferretone or Linatone can give your
ferrets loose stools.  No more than a few drops to one pump a day is
recommended, and it's not thought to be necessary to give them any at
all if you're using a good food.

Similarly, many people give their ferrets a small amount of a cat
hairball remedy such as Laxatone or Petromalt on a regular basis.
This can help them pass the styrofoam, rubber bands, and such that
they seem to love to eat, as well as helping to prevent hairballs from
fur swallowed during grooming.  Even better, most ferrets seem to
think of this as a wonderful treat, too.  As with all treats and
supplements, give them only in moderation; you can estimate how much
by taking the recommended cat dosage and adjusting for a ferret's
smaller weight.