That is true. For example MSG are added to most Chinese food to make it tasty but actually MSG isn't good for your health because it is an additive that is made chemically. Also juice and many other substances like food coloring are added to food to make it either yummier or prettier but will affect us after years of accumulation. Your question isn't very clear so what are you trying to ask exactly?
Tell the friend to be careful not to take too much. It does not sound like they have a negative relationship with this drug, nor serious health issues. Telling them to be cautious is the most helpful/appropriate thing to say here.
