Please be advised that there is a currently active thread on this running in ADD here, although the premise is a little different pharmacology and interactions are still discussed:
http://www.bluelight.ru/vb/threads/651672-Serotonin-and-LSD
Also there should be more threads on this in PD, I'll try to merge them for you and report back here.
edit: just mega-merged them and I'll merge this one in as well.
I'd like to get a nice bit of info to put in the OP for future reference, since there are a lot more people bound to ask this question.
I think this is interesting:
SSRI's can end trips if the increased synaptic serotonin is enough to outcompete the psychedelic bound to the 5-HT2a receptor. Obviously this is going to be a less effective strategy the more potent your psychedelic is. This is also partially why people on SSRI's find LSD and other psychedelics less effective (more serotonin in the synapse for LSD to compete with), although receptor downregulation/desensitization could also play a role.
Also as a nice sum up I think it can be said that results vary widely and
caution must be taken not to just take a large dose of acid, assuming the results will be dampened. Not every SSRI is created equal and anecdotal evidence suggests sometimes there is little or no reduction in effects and still a big potential for extreme states such as ego death.
Another point is that there are no guarantees that the quality of the trip will be a comfortable one by that I mean no different from the LSD alone... apparently some people don't take the interaction so well and there can be consequences to experienced mood or mental clarity that may be further magnified by the psychedelia.
Finally, be aware that taking LSD is fundamentally destabilizing to your mind or at least it has a fair potential for it (even if there is also potential for beneficial effects if there is proper reintegration and restabilization), so if you are taking SSRI - assuming this is for a reason - tripping may not be the smartest thing for you to do.
So the good news seems to be that there is no reason to suspect physical harm from the combination, but the warnings must be given that there is an element of unpredictability when it comes to intensity and possible non-descript mental effects on top of the psychedelic ones. There may be some data out there on the difference in result between specific SSRI's when combined with LSD, but I highly doubt this has been mapped.
How's that for putting something in the OP? Kindly help as I'm sure there are people who are more knowledgeable on this subject than I am who can formulate some elegantly worded advice. Thnx.