Risk of IM injection in anticoagulated people

Intramuscular (IM) injections are invasive procedures; bleeding and bruising may occur at the injection site. Although there is no definitive guidance, most IM injections are considered minimal to low-risk procedures.

Giving IM injections to people on oral anticoagulants may increase risk of bleeding or bruising at the injection site.

There have been occasional reports of compartment syndrome with injections into deeply located muscles.

Considerations for healthcare professionals

Before giving an IM injection to a person on stable oral anticoagulation therapy consider:

• if another route of administration is suitable
• if an alternative medicine or treatment is an option
• if there is a high risk of adverse effects (for example, bruising)
• if the injection affects anticoagulant control
• if the anticoagulant alters the effectiveness of the injection
• the implications of delaying the dose or temporarily stopping the anticoagulant
• if specialist advice is needed from the clinician managing anticoagulation

IM versus SC administration

The Green Book suggests the IM route is usually preferred for vaccines. This route is less likely to cause local reactions compared to deep subcutaneous (SC) injections and is more effective. There is moderate grade evidence that IM injection of vaccines gives a better immune response than SC injection.

For people with bleeding disorders, the Green Book suggests vaccines normally administered IM may be given by deep SC injection to reduce bleeding risk. However, extrapolating this advice to people taking oral anticoagulants may not be desirable and may be outside product licensing.

Before considering the SC route as an alternative, check:

• product licensing to determine if administration is ‘off-label’
• evidence of efficacy via SC route
• risk of adverse effects via SC route such as local skin reactions

Balancing risks and benefits

If a small volume IM injection is necessary, assess risks versus benefits for the individual.

Assessing risk

To identify people at high risk of bleeding, you may need to consider the person’s:

• other medications (especially those likely to increase bleeding risk, such as antiplatelets)
• renal and hepatic function
• past history of bleeding
• self-reported history of problems with previous IM injections

Using prescribing data

Check information relating to both the IM injection and the oral anticoagulant in resources such as:

• the Summary of Product Characteristics (SmPC) specifically checking contraindications (section 4.3), special warnings and precautions for use (section 4.4), interactions (section 4.5) and undesirable effects (section 4.8)
• Green Book for general and vaccine specific advice
• UKHSA immunisation information for healthcare professionals for vaccine specific advice

Be aware that warnings and advice in prescribing information can be variable and inconsistent. Some suggest caution, whilst others do not raise any particular concern.

If a risk is not mentioned in prescribing information, it does not mean there is no risk.

Seek specialist advice if needed.

Warfarin or acenocoumarol specific advice

A recent INR and information about INR control will help with decision making. There is no need for an extra INR check prior to injection.

Do not administer an IM injection if the INR is above the therapeutic range.

The SmPCs provide advice on continuing anticoagulation for surgery or invasive procedures. Anticoagulation may be continued where there is no risk of severe bleeding (providing INR is <2.5). Since IM injections are considered minimal to low-risk procedures, this advice can be extrapolated to IM injections.

Injection specific advice

Advice regarding use of IM injections in anticoagulated people mostly relates to administration of vaccines, specifically influenza (flu) and COVID-19. However, advice also applies to most small volume IM injections as bleeding risk is usually related to injection route rather than active ingredient.

Influenza (flu) vaccines

Give the flu vaccine via an IM injection in people stable on oral anticoagulants. Those on warfarin or acenocoumarol should be up-to-date with INR testing with the latest result within their therapeutic range.

Although some flu vaccines are licensed for SC administration, the Green Book advises the IM route is preferred. See IM versus SC administration above for further background.

COVID-19 vaccines

The Green Book Chapter 14a for COVID-19 vaccination and UKHSA COVID-19 vaccination information for Healthcare Professionals provide advice. Give the COVID-19 vaccine via IM injection in people stable on oral anticoagulants. People taking warfarin, should be up-to-date with scheduled INR testing with the last result within their therapeutic range.

Other vaccines

Check the relevant Green Book Chapter as well as the SmPC.

Practical advice for IM administration

If an IM injection is required and other options are unsuitable:

• consider administration before the next dose of anticoagulant rather than immediately after a dose (delaying anticoagulant dosing may be possible)
• administer into an upper extremity to allow easy access for manual compression, inspection of bleeding, and/or to apply pressure bandages if needed
• use a fine needle (23 or 25 gauge)
• apply firm pressure for at least 2 to 5 minutes after the injection
• advise the person to watch out for bleeding at or around the injection site and for signs of haematoma for example, pain and swelling

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