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Citalopram is an SSRI that is used to treat depression. It does not lead to a high and causes a wide range of negative psychological and physical effects when people attempt to abuse it and can lead to serotonin syndrome and cause suicidal ideation. Those with bipolar disorder tend to go into a mixed state or manic states when trying SSRI's.
Common drugs are those which are well known and widely used among the drug community. This doesn't necessarily mean they are safe, but it usually comes with a longer relative history of use in humans with which to establish a safety profile.
No interaction information for Citalopram