SS8282 - When combining medications with similar mechanisms of action and similar side effect profiles, those side effects can be additive. Risperdal (risperidone) and Seroquel (quetiapine) do have similar mechanisms of action, but their secondary effects are slightly different. That is, these two molecules primarily bind to (and block) the same receptor sites (ie. dopamine-D2 & serotonin-5-HT2), but Seroquel and Risperdal do not just bind to these receptors exclusively.

Risperdal secondarily binds to (and more weakly to) alpha-adrenergic-1, alpha-adrenergic-2, dopamine-D1, histamine-H1 receptors, serotonin-5-HT1A, serotonin-5-HT1C, serotonin-5-HT1D, and sigma receptors.

Seroquel secondarily binds to alpha-adrenergic-1, alpha-adrenergic-2, dopamine-D1 and serotonin-5-HT1A receptors like Risperdal; although they do differ in the binding affinities (ie. the two drugs differ in the strength with which the bind to these receptors). Unlike Risperdal, Seroquel does not have any appreciable affinity for serotonin-5-HT1C, serotonin-5-HT1D, or sigma receptors.

It is the difference in the binding profiles, both in strength of binding to the various receptors, as well as binding to totally different receptors, that result in the differences in side effects between the two drugs.

When taken simulanteously, there are overlaps in both primary and secondary receptor binding profiles. Where the two drugs' binding profiles overlap the side effects that are a result of this binding are often additive. Both Risperdal and Seroquel significantly block histamine-H1 receptors, whose main effect is somnolence (ie. drowsiness), as well as possibly contributing to the drying effect that you are experiencing. These side effects are additive.

**** BTW - I want to confess that my last post to you contained an error. I said that the drying effect was due to blocking cholinergic receptors (which is what happens when this receptor is blocked). I looked up the receptor-binding profile of Risperdal today, and discovered that Risperdal has no affinity to cholinergic receptors. I have fix the other post by replacing anticholinergic receptors with antihistaminic receptors; both of these receptors cause similar side effects and the mistake was a mental lapse). Cholinergic receptor blockade has a much stronger drying effect than histamine-H1 receptor blockade, thus I automatically wrote anticholinergic, rather than antihistaminic without looking up a reference (as I should have). Sorry for any inconvenience. ****

So, in answer to your question, it is possible that taking Seroquel with Risperdal will probably increase drowsiness and a feeling of "sluggishness", and may contribute some to the drying effects that you are experiencing; both most likely mediated to the blocking of the histamine-H1 receptor. I would also be willing to hazard a guess that your body may be extra sensitive to the blockade of this receptor.

I hope that this corrects and clears up some of my unintended muddying of this topic. - Cam