The subcutaneous (SC, SQ) route is one of the most versatile routes of administration
in that it can be used for both short term and very long term therapies. The
injection of a drug or the implantation of a device beneath the surface of the
skin is made in the loose interstitial tissues of the upper arm, the anterior
surface of the thigh, or the lower portion of the abdomen. The upper back also
can be used as a site of subcutaneous administration. The site of injection
is usually rotated when injections are frequently given. The maximum amount
of medication that can be subcutaneously injected is about 2 ml. Needles are
generally 3/8 to 1 inch in length and 24 to 27 gauge.
Absorption of drugs from the subcutaneous tissue is influenced by the same
factors that determine the rate of absorption from intramuscular sites; however,
the vascularity in the subcutaneous tissue is less than that of muscle tissue,
and therefore absorption may be slower than after intramuscular administration.
But absorption after subcutaneous administration is generally more rapid and
predictable than after oral administration. There are several ways to change
the absorption rate:
- use heat or massage the site to increase the absorption
rates of many drugs.
- co-administer vasodilators or hyaluronidase to increase
absorption rates of some drugs. Conversely, epinephrine decreases blood flow
which can decrease the absorption rate.
Many different solution and suspension formulations are given subcutaneously.
Heparin and insulin are the most important drugs routinely administered by this
route. Drugs that are administered by the route must be soluble and potent in
small concentrations since only a very small volume can be injected. The rate
of drug release is controlled by the same factors used for intramuscular formulations:
slowly soluble salt forms, suspensions versus solutions, differences in particle
size, viscosity of the injection vehicle, etc.
In spite of the advantages of this route of administration, there are some
precautions to observe.
- Drugs which are irritating or those in very viscous
(thick) suspensions may produce induration, sloughing, or abscess formation
and may be painful to the patient.
- Irritating drugs and vasoconstrictors can lead to
abscesses or necrosis.
- If frequent injections are required, the injection
sites must be rotated.
One of the most obvious ways to achieve very long term drug release is to place
the drug in a delivery system or device that can be implanted into a body tissue.
The subcutaneous tissue is the ideal site for implantation of such devices.
Implantation often requires a surgical procedure or a specialized injection
device. The fact that the device will be in constant contact with the subcutaneous
tissue requires that the device materials be biocompatible (e.g., not irritating)
and do not promote infection or sterile abscess. Another advantage is that the
device can be easily removed if necessary. Examples of implantable devices are
Norplant®, Oreton®, Percorten® and an osmotically driven mini-pump
(Alzet®) which can deliver drug solutions for up to twenty-one days. Other
devices include degradable microspheres, vapor pressure devices for morphine
release, osmotic pressure devices to deliver insulin, and magnetically or ultrasonically
Sometimes ports and pumps are placed in the subcutaneous space and the attached
delivery catheter is placed in a vein, cavity, or artery.