Serotonin reuptake inhibitors are contraindicated in patients with bipolar disorder.

Quoting Wikipedia:

"In the 1960s, Benadryl (diphenhydramine) was found to inhibit reuptake of the neurotransmitter serotonin.[1] This discovery led to a search for viable antidepressants with similar structures and fewer side-effects, culminating in the invention of fluoxetine (Prozac), a selective serotonin reuptake inhibitor (SSRI).[1][2]"

[1]: History of Modern Psychopharmacology: A Personal View With an Emphasis on Antidepressants
[2]: Serotonin Reuptake Inhibition by Diphenhydramine Can Result in Serotonin Syndrome

For the aforementioned reason, its use should be limited in patients with bipolar disorder. Here is one more additional reference, Tanaka 2011: Diphenhydramine overdose mimicking serotonin syndrome. Individuals with a known sensitivity to antidepressants should avoid it, as well as amitriptyline and trazodone.

Furthermore, Benadryl alters normal sleep architecture, this is why many report feeling groggy in the morning after taking this drug. I can't find a direct citation, but the following research abstract hints at what I'm referring to: Depression, sleep physiology, and antidepressant drugs.