A little fluorescein in water.
Manufacturers should add something like fluorescein to all pills that are usable as date rape drugs or poisons, so that those become very visible in drinks and can't be used for crime any more. Fluorescein [using it as example] is extremely visible in either water or alcohol even at the very low concentrations even in the regular daylight (more visible in fluorescent lighting, and more visible yet in ultraviolet), and biologically safe to the point that it can be injected into your veins as staining agent for various sorts of diagnosis, at quite significant concentration and in a very visible amount (your blood goes green like alien's, under ultraviolet light at least. Cool). It is also very cheap. Only very small amount of fluorescein is required to stain a lot of water or alcohol bright green.
I used fluorescein as an example only because I have old jar of bath salt that's containing it (I also used fluorescein in lab at school). Quick check for safety of fluorescein turns up this. Apparently, for some eye diagnosis, fluorescein is commonly injected in concentrations that make you turn visibly yellowgreen and which make you piss bright yellow for a while. A milligram turns entire cup of water the very bright, highly unappetizing kind of yellowgreen which you can see in this photo. I figure if people are routinely injected with 500 milligrams of fluorescein for diagnosis, it would be quite safe to add 1 milligram of fluorescein to typical pill.
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