Doctors don't have test capabilities to know for sure but depending on one's symptoms they often prescribe serotonin reuptake inhibitors.
Side effects can include lowered libido, slower metabolism, quality of sleep circadian rhythms and gastrointestinal upsets.
Brands include Lexapro, Zoloft, Prozac, Luvox, Lovan, Aropax, Cipramil and the cheaper Alphapharm equivalents.
To get off them requires a bit of supplement > neurotransmitter juggling/testing.
Juxtaposed assistance can include a pet dog/parakeet, consistent exercise, omega 3 fish oil, Inositol, quitting sweets/alcohol/recreational drugs and inhaling (in confined spaces) paints, thinners and varnishes.