This is a bottle of Depo Medrol. It is one of the more commonly used long-acting cortisone injectable products. One of the more common uses is to give long-lasting relief from allergic itching. Allergies are a malfunction of the body's defenses, so you give enough cortisone to suppress the body's defenses a little.
I'm not crazy about using this in dogs. You inject a blob of this stuff into the dog's thigh muscle. The drug begins leaching into the bloodstream. The bigger the blob, the higher the blood level of drug. So, when you first give it, the blood level of drug is very high. As time goes by, the blob gets smaller and smaller, so the blood level gets lower and lower. Eventually, the blood level gets low enough that it won't suppress the itching anymore.
The problem is this: you never really know what the blood level actually is and how much is left. We know that the average dose in the average dog will take 8 weeks to be totally gone, but it could take longer, maybe 12 weeks or even longer. The problem comes in when the blood level of drug is too low to stop the itching, but there's still a significant amount of drug left on board. The dog starts itching again, so you give him another dose. That's the second peak on the graph. Now you've got the new dose, plus the leftovers. He's not itching, but you may have boosted his cortisone levels to an unsafe level.
So, what's unsafe about it? Aside from the effect on his liver and pancreas and adrenal glands and carbohydrate metabolism and skin, you might be suppressing his immune system.
Here's Stinky, the Shih T'zu. He doesn't look too bad from a distance (although this is a day after a good bath and trim). He (I do believe) had an allergy problem. His previous doctor prescribed tablets with a little cortisone, plus antihistamine. He also started in March giving him a stiff dose of Depo Medrol every two months. Some dogs might do okay with that. On the other hand...
Take a closer look at his legs. Stinky (like many dogs) has always harbored a few Demodex mites (a present from his mother). Unfortunately, when Stinky got a big enough load of Depo Medrol on board, it suppressed his immune system just a little too much and the mites took off like gangbusters. These legs don't look good, and this is two days after his bath, trim, and starting antibiotics for the secondary skin infection, and ivermectin for the mites.
These eyelids are what really hurt him, though. They were so painful that we had to anesthetize him to clean them up the first time. We can treat the mites, and we can treat the secondary Staph bacterial infection. The bad thing is this: it's only been three weeks since his last Depo Medrol injection. That means his immune system is going to stay suppressed for a while. We may not make much progress this first month. Drugs help, but we need his own body defenses working in order for him to get well.
There is a place for Depo Medrol in veterinary medicine. You need to be pretty sure what you're dealing with, though, because once it's in there, you can't take it back.