The advice applies even if a person has no symptoms.
It comes a few weeks after doctors discovered the virus in the sperm of an Italian man six months after he first had Zika symptoms.
Zika is spread in bodily fluids.
The main risk of catching the disease is from infected mosquitoes via bites.
Previously, WHO had said men without symptoms only needed to use condoms or abstain from sex for eight weeks as a precaution against spreading Zika.