Hello, Fentanyl may cause both vomiting and symptoms of faintness as it may make the blood pressure drop too low levels particularly if the starting blood pressure was on the low side. When prescribing potent opioids most doctors would try to prevent nausea and vomiting by adding drugs with anti-vomiting effects to the regimen. Metoclopramide is such a drug (for example). Nausea and vomiting on their own may cause sweating and faintness, so I can t say if it was fentanyl that directly caused the low blood pressure or its side effect (nausea/vomiting). If you had no itching or shortness of breath and you got better thereafter then it was probably a side effect. An allergic reaction with faintness would have gone worse without treatment. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr Panagiotis Zografakis, Internal Medicine Specialist