Grapefruit juice can raise the level of some medications in your blood, resulting in a potentially toxic effect. This was discovered after the juice was used to mask the taste of a medicine.
Taking medicine can be hazardous to your health. For example, certain antacids can prevent antibiotics from being properly absorbed into the bloodstream.
You should educate yourself so you know what active ingredients are in the prescription and over-the-counter medicines you are taking.