Which of the following drugs is known to have decreased absorption when taken with food?

Ciprofloxacin is known to have decreased absorption when taken with food, particularly when combined with dairy products or calcium-rich foods. This is due to the formation of chelates between ciprofloxacin and divalent or trivalent metal ions, such as calcium, which can reduce the drug's availability for absorption in the gastrointestinal tract. The presence of food can also slow down gastric emptying, further impacting the absorption of ciprofloxacin.

In contrast, phenobarbital, acetaminophen, and ibuprofen do not exhibit significant changes in their absorption profiles when ingested with food. For instance, phenobarbital absorption is generally not affected by food, acetaminophen absorption remains effective, and ibuprofen can actually benefit from being taken with food as it may help to minimize gastrointestinal irritation. Thus, ciprofloxacin stands out as the drug with notable decreased absorption when taken with food.