A 25-year-old man is brought to the Emergency Department by his friends. He is agitated, sweating profusely, and has dilated pupils. His heart rate is 130 bpm, BP 170/100 mmHg, and temperature 38.8°C. His friends report he attended a party and might have taken an unknown substance. He is now complaining of chest pain. An ECG shows sinus tachycardia but no acute ischaemic changes. What is the most likely substance responsible for his presentation?