Biodegrading ... into what?

"Biodegradable" is one of many words commonly used to indicate something is environmentally friendly, but like many technical terms, it often doesn't mean the same thing in common usage as it does to a scientist. To me, "biodegradable" just means that the substance in question can be processed by some biological route into some other, usually lower molecular weight, substance.

It says nothing about the toxicity or harm—or benefit—that might be caused by the new substance.

Now, stuff like dead plant and animal matter, and animal excrement, biodegrade as nutrients for a different species. There's generally not much that's harmful in that, with the exception of pathogens—and they're alive in their own right, not a biodegradation product, so I won't count them.

Some things biodegrade by design, others by their nature. Some stores are now using biodegradable plastic bags which are triggered by sunlight, and which break down over the course of a few years (instead of a few millennia) into CO₂, water, and biomass for the bacteria that eat them.

In terms of products (or packaging) advertised as biodegradable… well, if you put it in the trash, it won't biodegrade. You have to compost, not landfill, biodegradable stuff in order for it to actually become useful nutrients again instead of just trash taking up space. It's one of those concepts that isn't immediately obvious but makes sense once you think about it: landfills are designed to be stable, and biodegradation is inherently not stable, therefore landfills are designed so that stuff doesn't biodegrade.

There are other things which do biodegrade, however, whose degradation products are not always harmless. One of those is pharmaceuticals. Something as common as amoxicillin, which you've probably used at some point in your life, degrades into several different compounds in the environment, some of which may be toxic.

Many pharmaceuticals leave the body via urine or are simply flushed for disposal when expired, and aren't completely removed in sewage treatment plants and thus pass through into rivers or the ocean at the sewage outfall. Even 99% removal of a pharmaceutical can still result in detectable buildup in the environment, and many are removed at much lower rates, if at all. Pharmaceuticals are bioactive; it's what they do. Pharmaceutical biodegradation products are sometimes also bioactive.

There's a lot of work going on to determine just what sorts of compounds are likely to form, either in a treatment plant or in the environment downstream of one, and what the effects of those new compounds are. Until we know what they are, we can't test for their presence.