Though I use the term worming and the medicine is called a wormer, it is used as an aid to balance the gut flora of a guinea pig.
Like all grazing animals, guinea pigs simply eat off the ground and can ingest all manner of bacteria some of which is not good for the digestion process. It can result in a build up of alien gut flora which feeds upon the food which enters the gut but not processed for the benefit of the animal.
Guinea pigs should be wormed with Panacur sheep wormer at a dose rate of 0.4 repeated seven days later, once every three months.
The most usual symptom that there could be a problem is when an animal eats very well or even more than it used to but is loosing weight.