This is a good question.

Some drugs do not do well together mixed in the same bottle.
And some it is not an issue.

If you are using amber tight closure bottles from a pharmacy to store daily
meds, instead of those medication dispensers, it is better.

Electrolytes like potassium, sodium chloride, or magnesium chloride attract moisture and can contaminate other drugs and damage them with time, since they will attract moisture.

Things to look for:
changing color, swelling up (some pills may swell in size, become crumbly)
or sticking together. Any strange odor. Any one of these signals damage.
Aspirin, salsalate, will start to smell like vinegar for example, when damaged.
If you have capsules, do a test squeeze on them. They should be resilient. This pertains to real capsules, not caplets which are only shaped like capsules. Gelcaps like Tylenol are hard to begin with and are not "real capsules".
If they become hard as a rock with time...that signals moisture damage. Once moisture enters the picture, drugs degrade, change.

You can ask the pharmacy to save the little cylinder moisture pellets from their bottles. Drugs that are coated (like Wellbutrin), will leave no residue on them. So they can be recycled. They can be reactivated with a hair dryer to blow off moisture periodically, at the lower setting.

If any of your drugs has a terrible odor like spironolactone-- I would not keep it in a multi drug container.

All bottles should be tightly closed, and protected from light (amber plastic).

Vitamins are best kept separate...since they usually have an odor.
Antioxidants like Vit C, E are affected by light.