For anyone who has ever treated cattle, the question that must be answered every time is where the needle must go–under the skin or in the neck muscle? Although we simply read the label and do accordingly, seldom do we ask why some medications are one route while other medications are the other route. In addition, it makes a person wonder if one route is better than the other.
For those that don’t know, when cattle are treated with an injectable medication, the medication can be delivered one of three ways: intravenously (IV), which means in the vein; intramuscularly (IM), which means in the muscle; or subcutaneously (SQ), which means just under the skin of the animal. The majority of medications are given IM or SQ. These routes are established by the Food and Drug Administration (FDA) when the medication is approved.
Medications, be they vaccinations or drugs, are given in the neck. This is to save the better meat in the back and rump from being damaged by the injection itself. Note the medication is not unsafe, but any time an injection is given it will leave a scar. These scars are trimmed off the carcass and thrown away so they do not enter the food supply, not because there is drug residue in them but because most people would not find them tasty.
This leads to the first major difference between SQ and IM injections–since IM injections go into the muscle they cause more damage to the muscle tissue, leading to more meat being wasted. The norm used to be to label medications for IM usage, but since the waste issue was discovered in the 1980’s new medications are increasingly labeled for SQ administration.
This begs the question, “why are some medications still labeled IM if it is more wasteful?”. This has to do with the FDA approval process. Contrary to the image of huge corporations pushing reckless drugs onto the market, the FDA approval process is long and expensive. Medications have to pass several safety studies, including multiple year, long-term safety studies. They also must be proven effective at accomplishing the task they claim to. For example, if a vaccine claims to prevent pneumonia, then zero calves in every study that were given the vaccine can get pneumonia. This process typically takes 10 years and $60 million dollars.
Once the medication is approved it must be given exactly according to the label dose and route of administration unless under the supervision of a veterinarian. Consequently, the label is seldom changed because it would require an entire re-opening of the drug approval process. Since this would be quite costly, most medication manufacturers leave the label as is in spite of the meat savings.
The second major difference between SQ and IM injections is the rate of absorption of the medication. Because there are far more blood vessels in the muscle than under the skin, medications given IM are taken up faster into the blood stream. This can be seen as an advantage or disadvantage, depending on the desired effect. If the medication is an anesthesia drug, then faster is better. If it is a long-acting antibiotic, then SQ will allow therapeutic levels to remain in the animal for a greater duration. Uptake can be sped up or slowed by adding other elements to the medication, such as peanut oil to slow down uptake.
So which route is the best? The one on the label. That route has mountains of data on safety and efficacy behind it. Now I can finally sleep in peace.