Obviously it’s not eating. In some religions, it’s not eating or drinking, but for health reasons most fasters allow the drinking all the zero calorie liquids like coffee or tea. Zero calorie “diet” sodas with artificial sweeteners seem to have problems that as of yet are not fully understood, so I tell people to abstain from them. Sorry Diet Coke.
Fasting intermittently. So any fasting period interspersed with an eating period. People will quibble over definitions but I think any fast less than 48 hours I consider intermittent. I would consider intermittent fasting. Anything greater than 48 hours I would call short term or prolonged fasting. Short term fasting I consider 5 days (120 hours) or under, anything over that is prolonged fasting. Intermittent fasting includes time restricted feeding, alternate day diets, and modified alternate day diets. These categories are not absolute as many fasters will combine them. For example my favorite form of fasting is time restricted feeding, but I’ll occasionally skip my eating window thus incorporating in some alternate day fasting.
In the time restricted feeding variety of intermittent fasting, your eating window is the time you are allowed to eat (typically 1-8 hours). So with a 4 hour window you would consume all your calories within 4 hours every day (i.e. 6pm to 10pm). A 5 hour window you might do 5-10pm. The 8 hour window popularized by Martin Burkhan will often be a window of noon to 8pm thus giving the person both a lunch and dinner during the day. I think any eating window greater than 8 hours is really getting into the range of “normal eating” and isn’t really fasting.
I’m neither a medical doctor nor a pharmacist. However, early when I had started fasting I was prescribed an antibiotic for an infection and when I picked up my prescription the guy at the window said “take this with meals three times a day.” I said I’m fasting, do I have to take this with food? What’s the worst that can happen, a little nausea or am I going to get an ulcer?” They got the pharmacist and he said some people get some nausea if they take it on an empty stomach but there was no risk in me trying the drug without food to see how I respond.” I did and I was fine and took most of my medication outside my eating window on an empty stomach. So I think that’s what people should do. Tell their doctor or pharmacist they are doing intermittent fasting and see what they say. If it means you can’t fast, there’s always caloric restriction.