Subcutaneous and intramuscular medicines

Last updated: Tuesday, July 12, 2022

1. Subcutaneous (SC)

Administration of a subcutaneous injection
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. etonogestrel). This route can be an option 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
  • Being asked about mixing drugs in the same syringe, particularly in patients with terminal illness.
  • Checking the suitability of infusion fluids administered subcutaneously.

2. Intramuscular (IM)

Administration of an intramuscular injection
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. adrenaline 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 always 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 although there is some variation in the literature). Intramuscular injection of certain drugs may cause pain, abscesses or bleeding (e.g. NSAIDs, iron) so consider whether alternative methods of administration may be used. 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).