There are three major routes of parenteral administration.
- Intrevenous
- Intramuscular
- Subcutaneous
This will be a quick summary on the important aspects to consider in each type of injection.
First of all, how does drug absorption happen in each case?
In subcutaneous and intramuscular sites, absorption occurs by diffusion from the drug deposit to plasma. The absorption rate depends on the area of the capillary membranes absorbing the drug, and the solubility of that drug. When you administer a drug intravenously, you circumvent all the factors relevant to absorption.
Intravenous Administration
This method permits controlling drug delivery with an accuracy and immediacy not possible with any other procedure. It also permits the adjusting of drug dosage according to patient response, for example with anesthetics.
This the best, and many times the only method for administering irritating substances, since blood vessel walls are relatively insensitive, and the drug is greatly diluted by the blood.
There are also downsides to this, of course.
- Unfavorable reactions may occur, since you can obtain high concentrations of the drug in your blood and tissue quickly. It is reccomended that you administer the drug slowly, by infusion, rather than rapid injection, and that the patient's response should be monitored closely.
- Once you inject it there is no going back.
- Drugs that are in an oily matrix, precipitate some of the blood components, or screw up your red blood cells should not be administered by this route.
Subcutaneous Administration
This method is very often used. However, keep in mind it only works with substances that are not irritating to the tissue. If not, you run the risk of horrific pain, necrosis, and tissue sloughing.
The drug absorption rate using this method is rather slow and constant - making it able to provide a sustained effect.
There are some nifty tricks that can be used to vary the rate of absorption of the drug. A good example is insulin delivery. A suspension of insoluble insulin will slower absorbed slower in this method than the soluble preparation of insulin. You can also add a vasoconstrictor to the drug to make the capillaries around it absorb the drug slower.
Some hormones are administered subcutaneously in a solid pellet form. Absorption of these preparations occurs over a period of weeks or months.
Intramuscular Administration
Drugs in aqueous solutions can be absorbed by this method quickly. However, the absorption rate can vary depending on the blood flow to the injection site. Absorption can be modulated by massage, local heating, or exercise. You can slower the absorption of a drug administered intramuscularly by putting it in an oily matrix or in suspensions. This is done often with antibiotics.
A good example of blood flow modulation would be injecting insulin and running. If you inject the insulin to your thigh and then run, you'll probably get a sudden drop in blood sugar. The increase of blood flow to the leg due to running causes an increase in absorption. Injecting in the abdominal wall or arms before jogging could decrease this drastic drop in sugar levels.
The amount of fat covering the the muscular tissue is also important. When we woman are injected in our butt, for example, we absorb the drug slower than males because we tend to have more subcutaneous fat there than the men, and fat is relatively poorly perfused (fatty tissue doesn't have many blood vessels). In very obese or emaciated patients drug absorption can exhibit unusual patterns in both subcutaneous and intramuscular administration.
