I'm not a doctor so I can only speak for myself but in my experience the only respiratory problems I've had was from 'forgetting to breathe', but nothing like a pharmacological respiratory suppression. Eventually you will either remember to breathe or recall that breathing is autonomous and all will be well as long as you don't panic. You're not likely to fall asleep during the experience. One tip from Terrence McKenna, silly as it may sound at the moment, is to sing... which will help with respiration and with panic.
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