Every seven years, the adult Vulcan undergoes an extreme and erratic physical and psychological imbalance, which can be fatal if the mating ritual is not carried out. 

When Spock underwent Pon farr in 2267, the U.S.S. Enterprise journeyed to Vulcan, where his betrothed mate chose another champion to fight for her — Captain Kirk. Only after McCoy deceived Spock by injecting Kirk with a substance that simulated death did Spock return to his normal, logical demeanor.

Certain agents have been known to accelerate the Pon farr.  For example, T'Pol found herself undergoing the Pon farr prematurely after being exposed to foreign microbes on an away mission in 2153.