If you get a chance, check out Richard Baxter's Christian Directory.
There are different levels of fasting, with not taking any food or drink other than water being about the most extreme. Other forms can be to reduce the quantity and quality of food eaten. For example, eating one or two meals per day would qualify as a fast, although less rigorous than complete denial. For a person who must take medication and eat food, I would recommend doing one of the following:
[1] Eat meals only when medication needs be be taken (if medication needs be be taken 3 or more times per day, this would not work).
[2] Reduce the quantity of food eaten at meals - say cut the portions in half or eat only enough as needed by the medication.
[3] Limit the quality of food, say eat only plain bread and water for a the period of fasting.
Some quarters of the church have historically fasted weekly by not eating meat on Fridays. It is hard to find people who do this anymore.
Scott