Why is it important for you tell your physician or pharmacist all of the medicines you are taking?

I think it's because that way if the drugs have any affects on you, they will know what caused and may find a cure. And will warn other people like you not to use it...

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Many of the chemicals in prescription and non-prescription medications can react with each other or with foods you eat, so it is important that all your doctors and the pharmacies that fill your prescriptions are aware of all prescription medications, supplements, herbs, and over-the-counter medicines that you use. This will help them avoid giving you medications that should not be taken together or, because of a potential interaction, need you to take in a special way. They can also mark your records with important drug allergy information so that information is available to prevent a similar drug (or the same drug) from being dispensed to you.

Doctors also need to know what medicines or other remedies you may use to help them diagnose your problems. Some drugs or supplements can mask symptoms or be a cause of some symptoms or side effects and delay proper diagnosis and treatment.