Are haematoma possible?

Haematoma at the injection site are still possible even when using a needle-free system.
They might occur with patients taking anticoagulants to treat cardiovascular disease (e.g., Marcumar) or thrombocytic aggregation inhibitors (e.g., ASS100 = time-released aspirin), or patients occasionally taking aspirin for pain relief.
Small bruises can also be due to too little or too much pressure when positioning the injector on the gingiva.