Interfering with how the medicine is digested and absorbed by the body. For example, milk blocks iron absorption in the stomach. Calcium found in dairy products can bind some medications, such as iron supplements and some antibiotics (Cipro and Levaquin, a quinolone.)
Pros of elongated: much more comfortable for adults. I would definitely choose it for the master bathroom. Pros of round: easier for very young children to use due to the smaller opening in the seat. Also, there are code requirements of how much foot space you need in front of a toilet.
Elongated toilets with a commercial type seat. These are both made to hold heavier people and are found at most hardware stores. I'm not sure how heavy the person in question might be, but if you can, look for products such as bariatric toilet seats, which usually have a 500 + pound weight capacity, and be sure not to buy a wall-mounted unit. Porcelain units are actually recommended over stainless steel.