I think both Mark and mhr4 are correct in their own ways.

At one point facts lead to a decision or belief. I tend to post facts when I post mostly on the subject of supplements. But at times,
I enter the "belief" point of view when I have personal experience.

Here is an official monograph on 5-HTP, which comes from one of the sources that I respect:
http://www.umm.edu/altmed/articles/5...han-000283.htm

It contains a list of other drugs that may interact with 5-HTP at the end. It also contains the typical warnings that appear on many supplement advice sites, which may or may not happen.

To get a clearer picture as to why 5-HTP is more potent, here are the pathways that tryptophan is involved with:
http://www.genome.jp/kegg/pathway/map/map00380.html

Tryptophan is involved in many reactions. Each rectangle is an enzyme that converts the various substance in all directions from tryptophan. Notice that 5-HTP has a much more limited function.
One of the paths 5-HTP may take is the kyneurine pathway:
http://en.wikipedia.org/wiki/Kynurenine_pathway
This synthesizes niacin in the body, which is a good thing.
It also contributes to a negative inflammatory pathway which is still being investigated. Tryptophan but not 5-HTP eventually may be used by the body to make Acetyl CoA which is very important (lower left).

This graph is very complex and one does not need to understand it all...but it illustrates where things happen, and which substrates are used.

The way I look at 5-HTP is that it is more of a "drug" than supplement because its actions are more limited. At this time our knowledge of brain biochemistry is really limited, and we are finding that genetic differences between people determine how they respond to drugs, and also may need more or less of a certain nutrient. 5-HTP has potential to cause harm, but I have not read that it lives up to this potential commonly unless misused or used with other certain drugs. When 5-HTP came out it was ferociously expensive... up to $60 for a month's supply. It has come down since then. Tryptophan today is also expensive, and in some ways, this high price limits the potential for taking too much by most people. If one is going to use tryptophan modestly, I see no potential dangers. There is significant tryptophan in egg whites for example and it is present in high amounts in whey protein. When you get above 2grams a day, then supervision by a doctor is wise. Taking tryptophan ALONE may allow it to cross the BBB (blood brain barrier) more effectively so really high doses may not be needed for most people.

I'd also make sure when deciding to use 5-HTP you consult your doctor because of the drug interaction potential.

Serotonin syndrome really only became recognized with the introduction of SSRI drugs. These are far more likely when misused to cause serotonin syndrome. They even interact with DM (dextromethorphan) an OTC cough suppressant. The most common interaction is SSRI + migraine triptan use. But doctors still give these together. This is something that bothers me tremendously!