There are a number of conditions that can cause weakness and/or paralysis of the hind legs. I can give you some possibilities, but you'll need to see a vet for a definite diagnosis and proper treatment.
Scurvy, caused by insufficient vitamin C, can result in hind leg paralysis. Adequate vitamin C does eliminate scurvy. It takes about a week to see results. You would need to give 50 mgs. per day. Vitamins you put in the water aren't good. They makes the water taste bad, and the guinea pig won't drink enough. In addition, those pet store vitamins have often deteriorated before you ever get them. You need plain vitamin C, not a multiple vitamin formula. Human vitamins are best. Many guinea pigs will eat the orange-flavored tablets made for children. If you use 100mg. tablets, you'll have to cut them. You can also use the liquid vitamin C made for infants. Just put the appropriate amount on a treat.
It's common for injury to the spine, pelvis, or limbs to result in hind leg paralysis. Sometimes administering corticosteroids can help the guinea pig heal. Arthritis is another possible cause. It can be treated with Rimadyl and/or glucosamine supplementation. Degenerated or ruptured vertebral discs also cause this condition. Back injuries can occur very easily occur in guinea pigs. A twist the wrong way while playing can do it. X-rays would detect these conditions.
The sporozoan parasite, Encephalitozoon cuniculi, is yet another possibility. It is treated with anti-parasitic medications, such as febendazole. Some bacterial infections can also result in paralysis. There are no tests to detect these. Some vets simply treat with febendazole and a safe antibiotic in case these conditions are to blame. You may want to speak to your vet about that possibility.
Here's a site with much more information on hind leg paralysis and weakness. It's a rabbit site, but the information also applies to guinea pigs. Rabbits, however, manufacture vitamin C in their bodies, so there is no information on scurvy as a possible cause.http://www.bio.miami.edu/hare/paresis.html Link
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