Effects - Delirium from low dose of diphenhydramine

[mullet0224]

SWIM doesn't take diphenhydramine because it gives him weird dreams and something he believes is known as "sleep paralysis".
However, SWIMs little sister typically takes 2 50mg diphenhydramine "soft gels" to get to sleep every night. 100mg seems like a high dose to SWIM, but she's always done it.
However, last night she didn't have the 50mg soft gels, she only had 25mg pills. They were the wal-mart generic for Unisom I think. So SWIMs sister took 4 of them to sleep last night. Today she told SWIM after she took them she started shaking and hearing voices in her head whispering. She also hallucinated the door opening and a visible gust of wind blowing in, and then the door shutting. Apparently she was pretty freaked out by it.

SWIM has experienced effects just like that on diphenhydramine, but it was on much higher doses. Is there any reason why SWIMs sister could have experienced that? It seemed like a pretty low dose, a dose I wouldn't expect to induce delirium. Has that ever happened to anyone else at such a low dose?

If it makes a difference, SWIMs sister was also on heroin. But she is on heroin most other nights as well, and she takes 100mg diphenhydramine every night for sleep whether she's on heroin or not. Oh and she's 18 years old.