What makes this so difficult answer is that we are only just beginning in our understanding of the serotonin system and complications like serotonin syndrome.
Serotonin has been known since 1935, and its presence and the activity of several receptors in the nervous system were discovered in the late 1950s. It has been known since drugs were introduced that excessive amounts of anything can cause unpleasant symptoms - and serotonin-effecting drugs aren't any different, usually.
Lots of people see a term like 'serotonin syndrome' and assume that it must be complex and deep rooted. It's really not all that hard to understand once you get to what causes it, though.
Serotonin syndrome happens only when your body is forced into releasing too much serotonin* that it cannot break down or reuptake into neurons. The result is your body is totally overwhelmed and cannot use serotonin to effectively regulate the things it normally does - perception, muscle and vascular tone, and body temperature, among other things.
Serotonin syndrome is always considered to be incredibly unpleasant. It is very important to differentiate between bad experiences on serotonergic hallucinogens, or overdoses of stimulants like MDMA, and true serotonin syndrome. Many drugs that release serotonin or activate serotonin receptors can cause some of these symptoms, especially in overdose, but serotonin syndrome is in general more intense and requires medical attention.
This paper says you must have taken two drugs that elevate serotonin concentrations (releasers, MAOIs, reuptake inhibitors) and have one of the following symptoms (Hunter criteria) to be considered to have serotonin syndrome.
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spontaneous clonus - Twitching without a cause. (clonus = rapid, uncontrolled twitching of muscle groups)
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inducible clonus with agitation or diaphoresis - Twitching that is aggravated by stimulation, agitation, and uncontrollable sweating.
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ocular clonus with agitation or diaphoresis - Uncontrolled twitching of the eyes, and sweating. (This is not nystagmus, the fluttering of the eyes you get when on MDMA. This means your eyes cannot effectively track objects)
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tremor and hyperreflexia - Continuous trembling and shaking, and easily excited reflexes
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hypertonia, temperature above 100.4 degrees F (38 degrees C), and ocular or inducible clonus - Rigid muscles, high fever, and twitching of the eyes, or twitching of the body brought on by stimuli.
The famous case of serotonin syndrome that killled Libby Zion was pethidine (Demerol), a painkiller that releases serotonin, and phenelzine (Nardil), an irreversible MAOI antidepressant. The doctors at the hospital she visited were overworked, and were working on the false assumption Ms. Zion wasn't on medication at all.
Serotonin syndrome is seen with e.g. combination of MDMA and DXM, cocaine and tramadol, or MDMA and 5-MeO-DiPT. Generally, SSRIs will simply block the ability of drugs like MDMA to release serotonin. Either way it is a waste of your time taking MDxx and SSRI drugs!
5-HTP and/or tryptophan are not converted to centrally active serotonin and do not pose a risk for serotonin syndrome. Nor are direct serotonin agonists like psilocin, LSD, the NBOME series generally responsible for SS. All negative effects are not serotonin syndrome.
* If you have heard of something called a "hypertensive crisis", that's very similar. It's caused when you have too much norepinephrine (or adrenaline) released at once, with nowhere to go. This, again, usually happens with MAOIs or antidepressants, combined with stimulants. It can even happen at the same time as serotonin syndrome, making the problem worse.