H2 receptor antagonists (cimetidine, ranitidine) can interact with selective serotonin reuptake inhibitors (citalopram, escitalopram, fluoxetine). These interactions can lead to serotonin toxicity. This is rare, but still important to be aware of.
Case report of a syncopal episode associated with fluoxetine. (Link)
Parkinsonism associated with fluoxetine and cimetidine: a case report. (Link)
Serotonin toxicity: a short review of the literature and two case reports involving citalopram. (Link)
The effect of cimetidine or omeprazole on the pharmacokinetics of escitalopram in healthy subjects. (Link)