thank you for your query. there is no common drug interaction between doxycycline and duloxetine (cymbalta). however, duloxetine can increase the effect of cannabis on the central nervous system to some extent. however, that too appears unlikely with only a few puffs of marijuana. also, it does not explain the continuous muscle twitches since then. If I a patient with your symptoms comes to me, I would reassure him not to worry about drug interactions and heart attacks. however, in my opinion, it seems the heart beating fast (palpitations) and muscle twitches are manifestations of anxiety, due to a relapse or worsening of the primary symptoms for which duloxetine was prescribed. this is also supported by the fact that effects of duloxetine (cymbalta) has plummeted. I would have either increased the dosing of duloxetine, or top it up with another antidepressant or switch altogether to another antidepressant. please consult your treating psychiatrist for adjusting the dosage of your medications. hope i ve been able to satisfy your query. please don t hesitate to contact me for any further clarifications.