Subcutaneous route of drug administration, also called hypodermic administration, involves injecting a drug into the loose connective tissue between the dermis of the skin and the muscle layer. It is usually performed on the external side of the arm or thigh, or on the anterior face of the abdomen, and generally admits smaller injection volumes than intramuscular route.
Absorption from this route is fairly rapid from aqueous solutions, but slow and sustained from repository preparations. Subcutaneous route may be used for fairly insoluble suspension and for implantation of pallets.
Subcutaneous injections are administered at a 45 degree angle with a 25 to 26 guage needle (with diabetic needle gauges now available up to 31 guage) and a 3/8 – to 5/8-inch needle. In lean or obese patients, the injections should be administered closer to a 90 degree angle.
Special forms of subcutaneous route
Novel modalities of subcutaneous drug delivery include dermojets, pellet implantation, device implantation
1. Dermojet: Dermojets are high-velocity jets of drug solution projected into the subcutaneous tissue, enabling virtually painless administration. This projection of the drug is done at a very high velocity and so the drug passes through the various layers and finally, gets deposited in the subcutaneous tissue. Since there are no needles involved, this technique is suitable for mass injections.
2. Implantation of pellets: The drug is incorporated in form of solid pellets and is placed under the skin for sustained release over a long period of time.
3. Nonbiodegradable and biodegradable implants: Here, the crystalline drug is incorporated in tubes or capsules and planted under the skin like the pellets. The nonbiodegradable ones have to be removed after some time but biodegradable implants don’t pose this problem. The examples of drugs administered via this route are- the oral contraceptive levonorgestrel incorporated in silastic capsules for longer duration of action, insulin incorporated in mechanical pumps, local anesthetics, and vaccines which are preferred by this route as the active protein moiety directly reaches the lymphoid tissues through the lymphatics without any interference from any other enzyme.
Advantages of subcutaneous route of drug administration
1. It is a good route of administration especially in skin infections.
3. Absorption is slower thus, it is a good route if a prolonged effect is to be achieved.
4. Self-administration is possible as the injection need not be penetrated deeply.
5. Depot preparations for sustained action can be made.
Disadvantages of subcutaneous route of drug administration
1. If the drug is irritating it might cause the sloughing off of the skin epitheral tissue.
2. It is suitable only for nonirritant drugs.
3. Drug absorption is slow; hence it is not suitable for emergency.
4. Similar to the i.m. route, only small volumes can be administered.
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