Olan'vora was a Varro biochemical reaction. During and after mating, the epidermal skin luminesced. Psychological bonding increased with exposure, stimulating the areas of the brain responsible for love. If allowed to advance, the olan'vora became permanent and the couple became biochemically linked. Separation thereafter was rare, however, up until this point the effects would diminish in a matter of months with decreased exposure. Withdrawal symptoms could be unpleasant.
Harry Kim, who had sex with Derran Tal, a Varro female, experienced olan'vora in 2375, despite Tal's belief it wasn't possible for a Human to be affected by it.
Tal said good-bye to Kim and left him since the more time they spent together, the harder it was to part. (VOY: "The Disease")