What might lead to a very high prolactin in a blood test?

Prolactin: High prolactin can be from a number of drugs, especially anti-psychotics, anti-depressants and opioids. A prolactinoma (pituitary tumor) can do this. And of course pregnancy and lactation. A high prolactin can interfere with pituitary lh/fsh, and cause periods to stop, infertility, or decreased testosterone in men. You should talk to your doctor who performed the test.