Risperidone is a much stronger 5ht2a receptor antagonist then seroquel is.
I read that hospitals use 12mg of cyproheptadine for serotonin syndrome as noted below from the link
http://www.uptodate.com/contents/serotonin-syndrome
Cyproheptadine is available in 4 mg tablets or 2 mg/5 mL syrup [8]. When administered as an antidote for serotonin syndrome, an initial dose of 12 mg is recommended, followed by 2 mg every two hours until clinical response is seen. Cyproheptadine is only available in an oral form, but it may be crushed and given through a nasogastric tube.
Cyproheptadine may lead to sedation, but this effect is consistent with the goals of management [1]. Furthermore, as a nonspecific serotonin antagonist, cyproheptadine may produce transient hypotension due to the reversal of serotonin-mediated increases in vascular tone. Such hypotension usually responds to intravenous fluids. Cyproheptadine is rated category B for safety in pregnancy by the US Food and Drug administration (FDA) (table 5) [8].
Definitive evidence of cyproheptadine's effectiveness is lacking. A small study used PET scan to assess 5-HT2 blockade in two volunteers after taking cyproheptadine (12 mg and 18 mg per day for six days). At 12 mg/day, there was 85 percent blockade and at 18 mg/day there was over 95 percent blockade of 5-HT2 receptors in the prefrontal cortex [16]. In addition, many reports describe the successful use of cyproheptadine to treat serotonin syndrome [15,17-22]. The majority of these patients received cyproheptadine at an initial dose of 8 mg, while fewer patients responded to as little as 4 mg and some had no response to as much as 16 mg.
Other antidotes — Antipsychotic agents with 5-HT2A antagonist activity, such as olanzapine and chlorpromazine, have been considered for antidotal treatment, but their efficacy is unproven and we do NOT recommend their use [1]. Chlorpromazine can cause orthostatic hypotension, although this is generally not an issue with serotonin syndrome, in which hypertension is common. Chlorpromazine can also increase hyperthermia.
Treatment with propranolol, bromocriptine, or dantrolene is NOT recommended. Propranolol has a long duration of action, may cause prolonged hypotension, and can mask tachycardia that can be used to monitor the effectiveness of treatment. Bromocriptine, a serotonin agonist, may exacerbate serotonin syndrome [1]. Dantrolene has no effect on survival in animal models