It's really dose related. Low doses can work pretty well as an adjunct to your favorite opioid drugs.
25mg diphenhydramine - good for allergies or opioid potentiation, if you want to stay awake.
50-100mg - good for allergies and making you sleepy/sedated, also for combining with opioids if you want to nod out. Stick to lower doses if possible.
150-200mg - basically a knockout drug at this stage, you'll feel like a zombie.
At even higher doses (probably more than 400-500mg), which are not recommended, delerium starts to kick in, dilated pupils, you lose the ability to form memories effectively, incredibly strong dry mouth, hot flashes, you lose the ability to urinate, etc. - all typical anticholinergic poisoning symptoms. It's not worth even bothering with, because not everyone hallucinates either, sometimes people just feel incredibly uncomfortable. Even a horrible shroom trip is better than a "good" diphenhydramine experience.