Why do food items and medicines expire after sometime?
There are a number of reasons
- bacterial infection
- fungi infection
- Most fungi are not toxic in themselves but they release mycotoxins in the food, which are definitely toxic
- Maggot infection
- Other infections
- E.g. a potato will slowly attempt to develop into a plant, slowly consuming the tuber (the tuber being the potato you would like to eat).
- Unstable chemicals
- Mainly for medicines. Note however that (from health.harward.edu; thank you @Charles for pointing that out):
Most of what is known about drug expiration dates comes from a study conducted by the Food and Drug Administration at the request of the military. ... What they found from the study is 90% of more than 100 drugs, both prescription and over-the-counter, were perfectly good to use even 15 years after the expiration date ... So the expiration date doesn't really indicate a point at which the medication is no longer effective or has become unsafe to use.
The main reasons are probably the first two (bacterial and fungi infection). Note that the list is incomplete.
Degradation of substances and chemicals don't happen only thru oxydation but also many other factors like light, microorganisms and othwr stuff. The chemical substance will also suffer a degradation without any external factors, and some are more likely to go thru such processes than others.
Expiring food items is obvious: for fresh food, it's almost always due to bacterial or fungal decay (spoilage).
Oxidization and sunlight doesn't really ruin food; it might alter the taste or decrease its appeal, although it remains edible even by first-world standards. However, it might destroy vitamins and other nutrients.
Medicines are a different matter. Some chemical compounds are slightly unstable, with half-lives measured in years. Instability can cause the compounds to randomly break into subcompounds that are ineffective for treating the intended condition at best and poisonous at worst. Unlike food, it's imperative to keep medicines away from sunlight (and oxidation in some cases) as both of these can further promote instability and breakage into subcompounds.
On the comment that Charles posted about medicine shelf life being much longer than advertised: The reason why they underestimate the shelf life is that it's better to have people throwing out potentially OK medicine than to have it sitting on a shelf for 20 years, going bad and sickening people, and prompting numerous lawsuits against the manufacturer. This type of deliberate underestimation in the name of safety is common in engineering - usually with weight limits advertised below the real weight limit. For example, an elevator able to haul 6000 pounds might be advertised as only 2000 because the manufacturer knows there's always going to be a few idiots who disregard the posted limit.
Food expires because its growing. Constantly. Every ingredient you put into making specific foods has a Date when its time has ended. Just like our bodies as we age. We are one year closer to expireing. We Grow old and Our skin starts to Wrinkle. Its just a way of life. If nothing ever expired we would over populate everything. Not to mention, if our foods didnt expire they wouldnt break down in our Bodies making it hard to gain nutrients from. Ending in a result of Death. We need the foods to break down in order to survive.