This is correct. Multiple diseases are being treated by multiple doctors writing multiple prescriptions being filled (and this is key) at multiple pharmacies. Nobody can keep track of all this.
The ‘Poisonous Cocktail’ of Multiple Drugs
In addition to seeing several doctors, many older people use multiple pharmacies to buy prescriptions. There may be no single health professional who knows what they are taking and could alert them to dangerous combinations. This is especially true in places where chain stores have replaced independent pharmacies or when the patient’s drug plan requires that medications be ordered by mail.