You get quite a large margin for error actually, especially with fentanyl. The dosing is easy, its just measured in micrograms instead of milligrams. 75-100mcg is a good hit for pain relief, whereas 500mcg is the dose you would use to induce someone. If you gave 400 or 600 or whatever it wouldnt matter as you plan on supporting the persons airway by some method of ventilation anyway. As someone whose done anaesthetics I just wanted to point out its not that dangerous, especially at a young age. People that die under an anaesthetic usually die from the physiology of old age. i.e. they throw of a clot and have a heart attack or a pulmonary embolis or a stroke. Its very rare to get an allergic reaction, and they are usually easily managed as the person is usually tubed by this point anyway and some adrenaline easily administered. There are a few rare forms of allergic reaction to some paralysis agents such as malignant hyperthermia but you would have to be pritty unlucky.
