Subcutaneous and intramuscular medicines

Last updated: Sunday, July 05, 2015

1. Subcutaneous (SC)

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Administration of drugs and fluids by the subcutaneous route may be by direct injection (e.g. insulin, enoxaparin), by intermittent or continuous infusion (e.g. diamorphine, terbutaline) or by implantation (e.g. testosterone). This route can be used when intravenous access becomes difficult (e.g. elderly or restless patients) and is particularly useful in palliative care.

The subcutaneous compartment can accommodate large volumes of fluid. Subcutaneous administration of fluid (‘hypodermoclysis’) can be a useful method of hydration in patients with terminal illness or following a stroke. The rate of administration should not normally exceed 2 litres in 24 hours at a single site.

The subcutaneous route is normally well tolerated, but pain, oedema and bruising can occur. Irritant drugs should not be given subcutaneously (e.g. prochlorperazine).

Common clinical problems

  • Mixing drugs in the same syringe, particularly in patients with terminal illness.
  • How to administer infusion fluids subcutaneously.

2. Intramuscular (IM)

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Intramuscular injections are used:

  • To ensure compliance (e.g. depot antipsychotics).
  • When other routes are less effective (e.g. hydroxocobalamin).
  • When other routes may be dangerous (e.g. epinephrine for anaphylaxis).
  • When a prolonged duration of action is desired (e.g. depot medroxyprogesterone).
  • As a short-term alternative to intravenous administration (e.g. morphine).

The intramuscular route is not suitable if a rapid onset of action is required. It is also more uncomfortable for the patient and only small volumes can be given (typically no more than 3mL). IM injection of certain drugs may cause pain, abscesses or bleeding (e.g. NSAIDs, iron). The route is avoided in patients with increased bleeding risk (e.g. raised INR, low platelet count) to prevent injection site haemorrhage, and in those with decreased muscle mass.

Common clinical problems

  • Clinicians may ask you about the IM route as an alternative option to IV where the latter cannot be used. In practice you probably won’t be asked many questions about intramuscular drug administration although this route is more common in some clinical areas than others (notably mental health).