I take 40 mg. of citalopram daily. can i take a supplement with ashwagandha, lion's mane, huperzine a, and vit. d3?

You could: But that is an awful lot to start all at once. You wouldn't start 4 medications all together at once, i would not advise all those supplements at once either. Try to determine exactly what you expect each of these to do individually and then decide which seems most important to you and start that first. Get vita D levels before starting that one.