What happens to medicines that justifies their expiry dates? Or is this just a drug companies' ruse to?

The expiry date is the date beyond which the manufacturers are not confident that is at least 90% of the active ingredient remains in the medicine, given recommended storage conditions have been maintained. All complex organic compounds will deteriorate over time and are also affected by storage conditions.

When I was first in pharmacy, most tablets came without an expiry date and pharmacies had stocks of tablets ten or even over twenty years old !

The expiry date is assessed on the basis of storage and deterioration data the pharmaceutical companies have tested for. Generic forms of medicines are very much cheaper than patented, branded drugs, so the expiry date on these is sometimes more best cautious guess than experimental data, with a relatively short time between manufacture expiry ( often two to three years).

Expiry dates are agreed with the drug authorities. Because the authorities must take efficacy and patient safety very seriously, these dates are of necessity cautious.

Out of date medicine are rarely harmful or toxic in some way. They merely have less active ingredient, so will not work as well.

It has nothing to do with drug company profits.

Some will degrade over time- they are quite complex chemicals, so different ones behave differently. Probably a lot have never been studied.

Of course, beyond a medicine's expiry date, the legal protections for their use will be reduced, and there might not be any good data on how long it lasts, so it would be inadvisable to use them.

Some do deteriorate, usually they have storage temperature requirements, e.g. 4-25°C.

Many years ago, I worked for a pharmaceutical company and had to investigate the drug substance in a white pill that developed green spots. A stored sample had also discoloured, it still "worked" but customers don't like to see change in their drugs.

Others will keep forever, e.g Aspirin etc.