You should not drink coffee while fasting for blood work. Even black coffee can potentially skew the results because caffeine affects your metabolism and certain blood test markers, especially those related to blood sugar and cholesterol. Coffee also contains substances that may impact some blood test results, and it is a diuretic, which can cause dehydration and make blood collection more difficult. During a fasting period before blood work, only plain water is recommended to avoid inaccurate test results. If you accidentally drink coffee, inform your healthcare provider as this may affect the interpretation of your results or require rescheduling the test.