If you have followed some posts I have made on prescription drugs you should know this kind of unexpected finding is very common. Most approved drugs have several kinds of actions beyond listed indications. This is most striking when you find psychotropic drugs with antibacterial or antiviral activity, as in the case of
fluoxetine. This brings us full circle from the first discoveries of antidepressant action in drugs intended to treat TB.
There is a more serious confusion over the role of receptors targeted by drugs. Few have only one specific function. About 90% of serotonin receptors are outside the brain. Within the brain, about 90% of serotonin receptors are typically inactivated by glial blockade. (Glial cells sit over these receptors on neurons, preventing them from functioning in synapses. In extreme cases synaptic stripping caused by intruding glia is associated with neurological degeneration.)
I have suspicions that common SNRIs also have antiviral activity. At this stage the research literature is confusing. Let me know if you run across an example. This might give us a clue about what doctors have actually been treating with them.
Receptors not only handle chemical signals between neurons and identify pathogens, they also facilitate communication between cells of the immune system. The effects of common drugs on these signals are little known.