The Coroners Office of Ontario described 110 cases of suffocation by plastic bags over a four year period. Diphenhydramine was the most common drug found at autopsy, and no cases using barbituates were reported.
Here's the
link
OTC meds can clearly do the job, it's just a matter of chosing the best option available.
Okay true so I was wrong about barbiturates being the only drug that you can do it, but I think you forgot 2 important parts from the same article you posted:
Benzodiazepines, diphenhydramine and antidepressants were the most common drugs found, with diphenhydramine the most common drug present at an elevated concentration.
It says diphenhydramine was the most common drug present,
does not mean that it was the only drug present. It also includes Benzodiazepines. Now benzos I can buy you could probably get away with CTBing on in certain situations. Apparently people have died drowning in their bath tub overdosing on benzos and alcohol (yes I'm only just remembering this ;>___>), but even that method I consider rather unreliable unless if it was a lake deeper than your standing height.
Benzos you definitely cannot get OTC, same with antidepressants, and the latter I find unreliable for with any kind of method. Urgo, OTC meds are useless on their own for trying to CTB with. Combining with prescription or illegal drugs? Sure I guess you can, but with diphenhydramine in particular, you're only gonna increase your odds of vomiting, with the drawbacks likely outweighing the benefits the drug can give. You'd be better off using it as an additional antiemetic (i.e. don't overdose on it) in combination with another more powerful antiemetic like metoclopramide, domperidone, zofran or olanazpine.
Edit: Okay so various sources (including Wikipedia) say there apparently is a lethal dosage for diphenhydramine, I still don't trust it because if it was still that easy to CTB over an OTC drug, we'd be seeing way more suicide reports with those drugs than we currently have.