Anne Chappelle is a board certified toxicologist with two decades of experience studying the chemicals that can kill people. In this video, Dr. Chappelle explains how eyedrops have been used to kill someone, why it's not true that pencils can give you lead poisoning, and other curious facts about poisons.
And if you're seeking more detail on Visine as a weapon, the following is from a 2013 Wired article on the matter:
Eye-drop poisoning is more routine you might think. Remember the Ohio man arrested last year for sending his father to the hospital by putting two bottles of Visine into his milk? The Pennsylvania woman who'd been sneaking Visine into her boyfriend's drinking water for three years? (The poor man suffered all that time with nausea, breathing and blood pressure problems). Oh, and let's not forget the Wyoming teenager who was angry with her step-mother; the girl just pleaded no contest to aggravated assault…
the active ingredient in these products is a compound called tetrahydrozoline, which turns to be a neat little arrangement of carbon, hydrogen, plus a little nitrogen. Or to be more specific it has the chemical formula: C13H16N2. It belongs to a family of compounds known for their ability to induce chemical reactions that either relax or constrict blood vessels. The former tend to end up in medications used to reduce blood pressure. The latter, which includes tetrahydrozoline, often go into nasal sprays or in eye drop formulas that are designed to "get the red out."
This is not, by the way, a simple blood vessel squeeze. It derives from the way these compounds bind to receptors in the sympathetic nervous system, altering signals to the vascular system, triggering the change. It's this action on the nervous system which puts tetrahydrozoline in the "neurotoxic" category on the Material Safety Data Sheet required of all manufactured chemical compounds.