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Hi there!
It is true that THC (and CBD) can impede the metabolism of many drugs, including Cialis, that may possibly lead to an overly-high serum level of Cialis, which can increase the risk of priapism (unresolving erection). However, that would take a very high amount of THC—much higher than the amount you should be taking for most reasons. However, it wouldn’t be a bad idea to lower the dose of Cialis to see if the effect were sufficient.
The cytochrome p450 system accounts for 70 to 80 percent of the enzymes involved in drug clearance, making it the most important system for drug metabolism. Both THC and CBD are competitive inhibitors of three different enzymes in the cytochrome p450 system; the most significant enzyme of the P450 system, CYP3A4, is involved in the metabolism of THC (and CBD) and Cialis. This means that CBD and THC are competing for the same receptors as is Cialis. So, when Cialis is taken concurrently with THC and/or CBD, Cialis is hindered from being metabolized, so serum concentration could possibly rise (and vice-versa)
https://ghr.nlm.nih.gov/primer/genefamily/cytochromep450
I hope this helps!
drkim