 Selective 5-HT2A receptor agonists like the 25-NB compounds can cause serotonin syndrome-like adverse effects such as hyperthermia, hyperpyrexia, tachycardia, hypertension, clonus, seizures, agitation, aggression, and hallucinations, most importantly in overdose.In severe cases, these effects can be fatal.In contrast, for reasons that are unknown, LSD, which is an agonist of both the 5-HT2A receptor and many other serotonin receptors, has never been associated with serotonin syndrome in more than 50 years of use.