Which dietary habit change is most likely to cause an adverse reaction to a medication?

Increasing the intake of grapefruit juice is most likely to cause an adverse reaction to a medication. Grapefruit juice can interact with certain medications by affecting the way the body processes the medication, leading to either increased or decreased levels of the drug in the bloodstream. This interaction can result in either reduced effectiveness or increased side effects of the medication. It is important to be cautious about consuming grapefruit juice if taking medications that can interact with it.

The other options, increased fiber intake, reduced alcohol intake, and reduced intake of citrus fruits, are not typically known to cause adverse reactions to medications like grapefruit juice does.