If the idea of inoculation or vaccination is present as well, you may want to protect yourself from intrusive emotions or unwanted urges.
Injections imply healing and protection, so this dream may be trying to show you how to become emotionally fit. Bear in mind that the syringe is also a phallic symbol, so your dream may have sexual connotations. Although much disliked and feared, the syringe is generally a benign symbol in dreams that work for your benefit. It emphasizes the message that in life, sometimes a little hardship and pain is needed for the greater good. [1]