SNRIs like venlafaxine and duloxetine raise serotonin in much the same way SSRIs do (they also raise noradrenaline, hence the extra "N"). For supplements, that means the same rules apply: most are fine, two are a hard stop, and a few sit in a cautious middle. The logic is the serotonin one. Anything that also raises serotonin can stack on top and push it too high.
The two hard stops
5-HTP is a direct building block of serotonin, so taking it with an SNRI piles serotonin on serotonin and risks serotonin syndrome, a real and occasionally dangerous reaction. This is a do-not-combine, not a be-careful. St John's Wort is the second: it raises serotonin in its own right and also speeds up the clearance of many other medicines, which makes it doubly unsuitable. Both belong on the avoid list with an SNRI, exactly as they do with an SSRI.
The cautious middle
A few mood-and-stress supplements have enough serotonergic or mood activity to be worth a clinician's eye before you start: saffron, SAM-e and ashwagandha. They are not automatic exclusions like the first two, but they are worth raising with whoever prescribes your antidepressant rather than starting quietly.
What is genuinely fine
Omega-3, vitamin D and magnesium have no serotonin involvement and sit comfortably alongside an SNRI for their own reasons.
If the supplement you are weighing up is for mood or anxiety specifically, that is exactly the case to check rather than guess. The free checker assesses your exact combination and shows you which tier each pair falls into. The same serotonin rules, in more detail, are in the SSRI guide.