Sugar?

There are no undetectable poisons. Every lethal substance invariably leaves behind some traces of its existence, some byproduct of the toxic reaction, some bio-chemical footprint in the body. Of course, in order to find those traces a forensic scientist needs to look for them specifically. If they don't test for a particular compound, then that compound's traces will remain invisible.

Or so I hear. Is that iced tea sweet enough for you? Can I get you some more sugar?