(Note: An earlier thread on this topic was closed for violating the rule against discussion of current drug use. However, discussion of the pharmacological effects of illegal drugs that does not discuss current use is permissible, and the mods are cool with restarting the thread on that condition. See: http://boards.straightdope.com/sdmb/showthread.php?p=9661770#post9661770 )
Does LSD have any known interactions with (legal) prescription drugs, OTC medications, or other illegal substances? Inquiring minds want to know!
MAOI antidepressants can be fatal combined with LSD. Combining LSD with ecstasy (“candy-flipping”) can lead to very dangerous hypertensive situations, but careful attention to the safety requirements of both drugs can lead to safer trips.
Those are the only situations I know about that are dangerous, but IANAD and that list is almost certainly not exhaustive. Do not take my post to mean that those are the only dangerous interactions possible with LSD!
In terms of less dangerous interactions, SSRIs decrease the effects of LSD. Other illicit drugs have different interactions (cannabis dulls the come-up, heightens the peak, and prolongs the come-down; shrooms cause a cross-tolerance effect; etc.). This information comes from a reference I used while investigating illicit drugs for an abnormal psychology class and contains a lot lot lot of discussion of illegal activity, so I won’t link it here. PM me if you’re truly interested.
Edited to add: I tried to stay within the rules of the board with my post, but the line between complete information and discretion is a tough one to walk in this case. If I’ve stepped over that line, I apologize and humbly request my post be blanked.