Not really big on Roman knowledge, but your primary poisons in "medieval Europe" would be arsenic, nightshade, wolfsbane, hemlock, and foxglove. To call these poisons dangerous is, for the most part, a massive understatement. To save some a few readers some internal distress, let's just say pain. Lots of pain. Followed shortly by a relatively messy death. Or you can Google the symptoms of each type of poisoning.

There are legends of ground glass being used, but that's likely a falsehood--any glass ground fine enough to not be noticed in the mouth will pass harmlessly through the body. Any glass which would cause intestinal bleeding would tear the gums and tongue.

There's also legends of diamonds being used as poison. This was because there was a belief that diamonds contained arsenic. It's far more likely that diamond powder (and ground glass, for that matter) were either terms for arsenic or were combined with it.

Basically, poisons worked, they were prevalent because they were easily-found and, in some cases, almost undetectable. Hence the position of food taster.