Autoinjector: A single-use pre-filled device with a fixed amount of drug. The patient applies pressure to the injection site or presses a button to trigger the needle to inject. The needle is never visible as it is always protected by a a shield. (e.g. include treatment for RA, MS, Epi-Pens)
Pen/ Insulin Pen: The patient is able to set a dose depending on medication requirement. Patient is required to activate a push-button, which triggers the needle to inject. Pens are normally reusable and have a replaceable needle. Often used for insulin
Pre-filled syringe: A single-use syringe pre-filled with a fixed amount of drug with a cap over the needle - the needle is visible when the cap is removed.