Befiradol (F-13,640; NLX-112) is a very potent and highly selective 5-HT1A receptor full agonist. It has powerful analgesic and antiallodynic effects comparable to those of high doses of opioid painkillers, but with fewer and less prominent side effects, as well as little or no development of tolerance with repeated use. A SAR study revealed that replacement of the dihalophenyl moiety by 3-benzothienyl increases maximal efficacy from 84% to 124% (Ki=2.7 nM).