Ok! It sounds like your sister's dog has stretched or ruptured his cranial cruciate ligament.
This is something humans get as well, but in humans it is called Anterior Cruciate Ligament.
The cranial cruciate ligament is one of the structures that stabilizes the knee joint. It is prone to problems, especially in overweight dogs. If the dog is in motion, and the lower leg is held still (by going down a hole, getting caught on a string, etc) while the upper leg keeps moving forward, there will be damage to this cranial cruciate ligament.
The symptoms seen are usually a sudden onset of hind-end lameness, with toe touching seen at standing and walking, but the dog often carries the leg when running.
Here is more information about this problem:
And you can see more here in this video:
Now, you were asking about Rimadyl.
Rimadyl is a non-steroidal anti-inflammatory drug (NSAID).
Aspirin is probably the best known drug in this class of drugs. Rimadyl, however, is much safer than aspirin, and much more effective (and expensive, unfortunately).
Here is more about Rimadyl:
So, there is NO over the counter equivalent for Rimadyl, unfortunately.
If your sister's dog has injured her cruciate ligament, she needs to be very stricly rested. If she has merely stretched it, then rest may allow it to heal. If it has ruptured, if she is over 25lbs, then surgery would be the best option for her.
Be particularly careful with her if she has to go UP stairs as this is when all her weight is on the back legs and she is more likely to do additional damage.
To help your dog to get up the stairs at night, I would recommend using a "sling" that you can make yourself.
Take a long bath towel, and fold it 3 or 4 times lengthwise, so that you have a long and thin shape. Put this under her belly, as far back as possible, so it is under her hips. Now, bring the ends up over her back so that you can hold the ends above her. With this, you can lift almost all her weight by using this sling, and thus really help her up the stairs. She can walk herself, with you right beside her taking her weight on the sling to help her. This is a really good way to help her up stairs or up into the car - or any time she has to go UP!
Alternatively, you could consider the “Bottoms Up” leash to help her with stairs:
I understand that you may have financial concerns as so many people do in this economy. I can give you some suggestions for where to turn for financial aid.
I'd start with the local animal shelters to see if they know of any low cost or subsidized vet care in your area.
And this site has a very comprehensive list of financial resources:
Of these, the most reliable one in terms of providing assistance is www.carecredit.com. I have heard very good reports about them!
I hope that this helps you to help your sister's dog!
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The above is given for information only. Although I am a licensed veterinarian, I cannot legally prescribe medicines or diagnose your pet's condition without performing a physical exam. If you have concerns about your pet I would strongly advise contacting your regular veterinarian.