Administration of medicine by injection is often referred to as giving by the parenteral route (this means that the treatment does not enter the body via the gut). Effective administration of medicine is a key part of most veterinary treatments and many medications are most effective when given by injection. Administration of medicine by injection is essential for some drugs that are destroyed by acids in the stomach, eg insulin.
Injections can be given into:
tissue under the skin (subcutaneous)
body cavities (abdominal ie intraperitoneal or thoracic, ie intrapleural)
This Factsheet will only consider the intramuscular and subcutaneous routes as these are the techniques most likely to be encountered by cat owners.
Aseptic techniques should always applied when administering injections. If the coat is very dirty it should be clipped and cleaned. Skin should be swabbed with alcohol. Never administer an injection through dirty or infected skin.
Different formulations of injection are used for the different routes and it is particularly important not to administer an injection directly into the blood unless it is specifically recommended for this route.
This is the route used for administration of most injections and vaccinations. Domestic animals have plenty of loose skin so it is very simple to lift a flap of skin and insert a needle into subcutaneous tissue. There are few important (or easily damaged) structures under the skin so this is a very safe route of medicine administration, and owners can easily be taught to give injections in this way. Owners of diabetic animals are taught how to administer insulin subcutaneously so that they can give regular injections to their pet at home. This route is unsuitable for administration of irritant medications as they may cause skin necrosis and sloughing.
Drugs given into muscles are absorbed very quickly because there is a good blood supply to this area. Injection into muscle tissue is not without risk since there are many important structures, eg arteries, veins and nerves running through the muscle tissue. It is important to check that the needle has not accidentally been placed in a blood vessel (particularly an artery) in the muscle, before giving the injection. Once the needle has been inserted into the muscle gentle suction should be applied to the syringe to ensure that blood does not flow back into the needle.
One of the disadvantages of giving treatment deep into muscle tissue is that it is difficult to know what is going on at the injection site after the injection has been given. Traumatic technique can result in bruising and, in animals with a bleeding disorder (eg anticoagulant rodenticide poisoning), haemorrhage can be severe after intramuscular injection. However, since the site of injection is hidden these complications may go unnoticed for hours or days.
Appropriate sites for intramuscular injection are the quadriceps (on the front of the thigh), lumbodorsal muscles (either side of the lumbar vertebral spinous processes) or the triceps muscle (behind the humerus in the front leg). The hamstrings (at the back of the thigh) must be avoided due to the possibility of damage to the sciatic nerve. Volumes of injection should not be more than 2 ml in cats. A positive action for insertion of needles into muscles reduces muscle damage and pain and massaging the site after injection disperses the injection and may help to reduce pain.
Some drugs are specially formulated so that they are more slowly absorbed and can sit in the muscle – being absorbed gradually over many months (so called ‘depot injections’).
Alternative routes of medicine administration
Respiratory tract The lining of the respiratory tract is thin and vascular and absorption of some drugs from this site can be very rapid. Intranasal vaccines, eg for kennel cough are given into the nose and the live virus is able to penetrate the lining of the nose. Nebulised drugs can be used for the treatment of respiratory disease and administration by this route allows rapid penetration to the local site where they are to have their action. However, the use of nebulised drugs means that the patient must breathe air containing the drug. Nebulised drugs can be delivered via a face mask although many conscious patients will not tolerate a face mask, so a special nebulisation chamber is often required. In cardiac arrest, adrenaline is sometimes administered through the endotracheal tube and absorption via this route is rapid and the technique is much safer than the alternative, intracardiac adrenaline injection.