Keep in mind I got this info from cursory google searches and back of the napkin math.
Tetrahydrozoline has an oral LD50 of 785mg/kg in rats. Meaning if you give 785mg for each kg that an individual weighs (ie 1570mg for a 2kg individual), this is enough to kill in 50% of cases.
If we extrapolate this number to a say 150lbs person (68kg) then they would require 53,380 mg, or 53 grams.
Visine, an over-the-counter Tetrahydrozoline product contains 0.5mg/mL in bottles of 15mL. So 7.5mg of Tetrahydrozoline in a bottle. And a 150lbs individual needs 53,380mg to reach the LD50 dosage. That means you need to take something like 7 thousand bottles.
Now, I am aware of cases where people have been poisoned by Tetrahydrozoline without realizing, and I can't imagine slipping 7k bottles of Visine in someone's drink lol. I don't really know what to make of this. The LD50 is stated by the manufacturers themselves, on regulated Safety Data Sheets. Maybe you can find Tetrahydrozoline products at much higher concentration than 0.5mg/mL but I couldn't find any.
TL;DR: If you take my data at face value though, ctb with Tetrahydrozoline is very unlikely to work.