Okay, firstly, ondansetron, mianserin and serotonin syndrome/toxicity (SS/ST). The product info leaflet list SS/ST because the muppets at medicine regulators such as the FDA, WHO and I assume the UK's Medicines and Healthcare products Regulatory Agency wouldn't know serotonin syndrome if it slapped them across the face with a halibut. This isn't my opinion, but that of SS/ST experts such as Ken Gillman and others:
Demystifying serotonin syndrome (or serotonin toxicity) (
PDF, p724)
The 'setrons aren't the only meds for which the muppets have gotten SS/ST badly wrong. Worse when presented with the evidence of their error the usual response is to dig their heels in, block their ears and sing LALALALA.
Bottom line: Ondansetron can't trigger SS/ST because it has no known impact on serotonin transporters which is the central requirement to induce SS/ST.
Mianserin also can't trigger SS/ST because it is an extremely weak inhibitor of serotonin reuptake. In the past mianserin was often prescribe together with SSRIs, just as its analogue mirtazapine is now. The ability of a drug to bind with receptors and transporter molecules is measured in Ki units which are based on the amount of the drug needed to saturate receptors/transporters. The less drug required the lower the Ki. Mianserin's value for the serotonin transporters (SERT) is about 4,000 Ki. To put that into perspective the value for the most potent SSRI, sertraline, is 0.21 Ki, and for the least potent, citalopram, it's 1.38 Ki. You would have problems physically swallowing enough mianserin to equal the serotonin reuptake blocking ability of a single sertraline dose, and even if you could SS/ST would be the least of your worries.
But this isn't the only reason mianserin can't induce SS/ST. The danger of the syndrome is the large spike in body temperature it causes. This can become high enough to kill. The potent 5-HT2A antagonists (blockers) cyproheptadine and chlorpromazine are the recommended treatments for preventing the temperature spike. Mianserin is also a fairly potent 5-HT2A antagonist which could counteract the temperature spike if ondansetron and mianserin were capable of triggering SS/ST.
Now for some potentially good news re ondansetron. I first heard of this med from my then psychiatrist who had been following its drug trials back in about 1990. All the indications were that it was a highly effective anti anxiety med, particularly for panic disorder, which not only had fewer side-effects than antidepressants, but could also ease nausea, a common anxiety symptom. So we both waited and waited for it to come onto the market, but it never did, at least not for anxiety. It seems the eyes of the Glaxo beancounters lit up like the Sydney Harbour News Years Eve fireworks when they realised how effective it was in blocking chemo and radiation therapy triggered nausea and how much cancer clinics were prepared to pay for it. While it was in patent it was selling for US$5 a tablet, wholesale!
It is still fairly expensive, though maybe not as much as mianserin, so while you may not be able to get it prescribed for anxiety at least you get to have a test run and hopefully enjoy a significant reduction in anxiety levels while you're on it.