When I was in college, back in the last century, I worked at a lot of different veterinary practices. There was the gruff old Midwestern cow doctor in boots and coveralls who smoked a cigar constantly, even during surgery. There was the fancy Colorado clinic that belonged to a former AVMA president and was staffed by a crew of cutting-edge young doctors. There was the neighborhood country vet who worked solo, without a secretary and only a part-time assistant, his waiting room crowded daily with everything from ducks to dogs to bunnies.
One thing all these practices had in common was that the doctors frequently prepared their own concoctions to dispense to patients. There were not that many approved veterinary formulations on the market, so if Minnie, the tiny teacup poodle, needed antibiotics and there was nothing on the shelf small enough to dose an itsy-bitsy dog, Doc would crush some pills and mix them in a bottle of something tasty, like Doggy-Yum Liver-flavored Liquid Vitamin and Mineral Potion. He'd tell you to shake it up well and give Minnie X number of drops every day. For Stinky, the golden retriever with the stubborn ear infection, he'd take a bottle of commercial ear medication, add a dollop of this, a syringe full of that. Abracadabra! The perfect ear remedy.
It was part of the old-timey charm of the profession for me. I liked the funnels, the brown glass apothecary bottles, the droppers. I liked the mysterious labels and colorful names. I've heard of potions named Pink Lady, Purple Thunder, Blue Death, Red Med, Texas Wound Lotion, Powerful Poof, and my favorite, The Green Bomb that Robby Fair taught me - a mixture we used for post-partum uterine infections in sheep and goats. But over the years, things changed. As companion animal medicine became big business, more commercial veterinary products came on the market and there was less call for us to make our own. In fact, we were legally obligated to use the approved stuff rather than our home-brews, unless we could medically justify eschewing the commercial product. But treating everything from tiny hamsters to half-ton horses, veterinarians continued to need to be creative, tailoring drugs to suit their patients in a process officially known as "compounding."
Compounding is the preparation of a customized medication to meet the needs of a particular patient. Compounding can involve mixing together several different medications to make administration simpler, like Old Doc's ear remedies. It can involve turning a single drug into a liquid or flavored form, so Mrs. I-Can't-Pill-My-Cat can get it down Old Tom. Compounding can be necessary for many reasons. For example, doxycycline is the drug of choice for cats with tick-borne diseases. Giving cats doxycycline in a tablet or capsule form can occasionally cause serious and permanent damage to the esophagus. That's bad. There is a liquid form of doxycycline on the market for human use. That's good. But if you use it to treat a cat you've got to force anywhere from ? to 1 teaspoon down Fluffy's gullet twice a day. For a month. Yeah, right. If we get doxycycline compounded in a stronger concentration with a nice fish or liver flavor, you can give much less. They still may not like it, but it's at least a little easier.
Then there are the human products we use that don't come in small enough sizes for our more petite pets. I love sending home a prescription labeled "Give 1/8 of a tablet three times a day." Have you ever tried to break a metronidazole tablet into eighths? It's tough, not to mention inaccurate. Much nicer to have it compounded. Then there are the human medications that come in appropriate concentration and liquid form, but the wrong flavor. Cherry or bubble-gum flavor may appeal to your kid, but the cat is usually not impressed.
Another time compounding comes in handy is for individual animals that need drugs that have gone off the market - epileptics who need potassium bromide to control their seizures; incontinent dogs who need diethylstilbestrol to keep from leaking urine all over the rugs; chronically constipated kitties who can't poop without their cisapride. All these drugs used to be readily available through human pharmacies, but were taken off the market over the years due to potential side effects in people, or simply because they were replaced by newer (and usually more expensive) drugs.
Over time, veterinarians turned to commercial "compounding pharmacies" to put together many of these drugs for them. These pharmacies also worked with human physicians creating individually tailored medications for people. For example, imagine you are allergic to one of the inactive ingredients used in making the commercially available form of a medication you need regularly. A compounding pharmacy can make up your meds free of the offending allergen.
As you can see, compounding is a good and necessary art. So if Minnie needs something compounded, we can just crush up her pills and mix it in something yummy, right? Well, no. Much as I enjoy the old-fashioned apothecary ambience, compounding is not as simple as it sounds. Tablets that do not taste too bad swallowed whole may taste incredibly yucky if crushed and mixed into a solution. If the tablet has a protective coating, breaking this down may alter the stability of the product. Mixing certain drugs with anything that alters the acidity even slightly, may cause the drug to degrade and become rapidly ineffective. When different drugs are mixed together, they can interact in such a way as to reduce stability or potency.
Risks of mixing
Some drugs loss effectiveness if mixed with anything water-based. Others are not readily soluble in water-based vehicles or flavorings and the drug will precipitate out. That can result in under-dosing when you use the top of the bottle, and overdosing when you use the bottom of the bottle, where all the active ingredient has settled. Some drugs lose efficacy if mixed with iron-containing liquids (like that Doggie-Yum Vitamin and Mineral Potion). Some antibiotics work perfectly if given in separate doses but rapidly become inactivated if mixed together. Even published "recipes" may not have been tested for stability, potency, and purity and even your local pharmacist may not have much experience with compounding.
The bottom line is, it's complicated. It can be done properly by Old Doc, and it's completely legal if done correctly, but veterinarians who choose to do a lot of compounding in their practices need to educate themselves carefully about all these interactions. I'm too busy (or too lazy) and rely on commercial compounding pharmacies instead. But even these companies vary in quality control and expertise. If you have compounded medications at home for Minnie, watch for signs of instability. Liquids should be discarded if you note any color change, cloudiness, precipitation, clumping or crystal formation, or if the container swells. With solid dose medications, watch for any unusual odor, cracks or chips in the tablets, swelling of tablets or capsules, or stickiness. Follow the label instructions carefully and if your medications don't seem to be working properly, tell Old Doc. We have to all work together, veterinarians, pharmacists, and pet owners, to assure that our custom medications are doing their intended job.