Medications: Prescription drugs are chemicals designed to produce specific and desired changes or functions within the body. But, as in the law of physics, for every action there is a reaction. That reaction may be drowsiness and/or a slowing of the person's reaction time. In the field of medicine these are identified as side effects and may effect, even seriously, a person's ability to drive.
A patient taking several different prescription drugs, particularly if they are prescribed by different doctors who don't have updated knowledge of other drugs being taken, may have even more serious side effects as each of the drugs creates its own side effects plus conflict with other drugs to cause even worse reactions. The latter is identified as polypharmacy.
Your parent's physician(s) can advise of the side effects of each drug plus the added conflicts through polypharmacy. You may also take all the prescription containers to a friendly pharmacist who can quickly do a computer-based analysis.
The American Medical Association has published a detailed report and recommendation to all of its physician members that they assist caregivers, answer their questions, and present their recommendations regarding the elder's physical and medical conditions. The report also recommends that the physician be actively involved in counseling the patient to hang up the car keys.