High doses of fentanyl injected quickly can result in overdoses that are atypical of opioid overdoses as most people know them, signalling a need to both be aware of the new symptoms and to tailor emergency responses. First responders and other front-line staff report that many overdoses caused by such injections are resulting in muscle rigidity, characterized by jaw and fist clenching, and a tightness to the chest that keeps it from rising and falling, making it difficult to ventilate the patient..