I have some sleeping problems, so I take a generic Benedryl to sleep. (It's the same active ingredient as Unisom, but costs significantly less.)
Last week was a very rough week for me. Within 3 days I took 1125 mg to help me sleep (very unusual to do so). I don't know if it was the same amount or not each time, but that means at least a minimum of 375 mg within a 4 hour period. That's 15 pills in 4 hours. The maximum recommended dose in 4 hours is 2 pills (50 mg).
Yes, it helped me sleep, but for the most part it did not phase me any more than taking 50 mg.

BY NO MEANS am I recommending this, just stating my personal experience. I think it was foolish and I hope never to do it again, but just saying, I'm plenty alive with no longer term side effects.
It did put me in a bad mood, but even 50 mg does that. I was severely depressed those several days. That could be a result of the medicine, or it could have been a combination of that, extreme lack of sunlight, my wife and son being away for the week, and other factors. But physically and mentally it did not affect me other than just helping me fall asleep.

AGAIN: I WOULD NOT RECOMMEND THIS

Just stating that I didn't die, and I'm a small guy (5'4" 120 lbs). Another person mentioned 750 mg I think, that that was recorded and someone survived. Still, I wouldn't try it.
A severe allergy, I'd say don't take more than 200 mg in 12 hours. The bottle says not to take more than 300 mg in 24 hours, so doing the math.... You have the FDA recommendations of 150, and my saying that probably 200 is OK. By no means am I the FDA, so don't do it based on my recommendation.
Disclaimer: I'm not the FDA. Don't listen to me. If it kills you, you can't sue me