Medications can be difficult. As you rightly point out, sometimes saying nois a way of gaining control. Other times the person simply does not understand the need. Also, some people have difficulty swallowing pills and this may increase as the Alzheimer's gets worse. Be aware that some people keep the pill in their cheek and spit it out when no one is looking.
First, I would ask your pharmacist if the medication is available in other forms. Sometimes a liquid is easier than a pill. Ask if it can be crushed and mixed in with something like ice cream or pudding that might be appealing. Also, make sure you have a medication that can be taken as few times per day as possible.
- With some people it is possible to use a behavioral approach, e.g., "When you have swallowed your pill, I have a dish of ice cream for you."
- When she refuses, back off and try again a little later.
- Try different approaches. This may have become her place to take a stand; if so, try to make it more casual and less of a power struggle.
- To the extent you can control it, pick times of the day when she is likely to be most agreeable.
Sometimes there is no good solution to this. As a caregiver, you can only try your best -- you may not be able to get her to take it every time. But if you can find ways to defuse the situation, you have a better chance.