A lot of people think "serotonin syndrome " is some sort of catch all that is just a Bad Thing on Drugs. What it really is, is an excess of serotonin being released into your body causing it to freak out and go hyperthermic.
The general recipe for serotonin syndrome is a powerful releasing agent (MDMA, tramadol, meth) mixed with a reuptake inhibitor or MAOI (that prevents the decompoisition of serotonin, like say DXM or moclobemide). Mixing several reyuptake inhibitors or releasing agents can trigger it too.
Unless you take a stupendous overdose of a single drug (many grams MDMA) you're not going to get Serotonin Syndrome but mixing powerul reuptake inhibitors, or huge doses of drugs (e.g. antidepressants) can cause it.
It's worthwhile noting that the reason you shouldn't mix, say, Adderall and DXM is because of a similar thing happening with another neurotransmitter, norepinpehrine. When too much norepinephrine is released and cannot be broken down, your hrart rate and blood pressure can rise to dangerous levels and it very frequently induces massive panic attacks (norepinephrine is a "fight or flight" horomone). The same "recipe" works here, releasing agent plus something that stops breakdown.
