I know you cant drink when taking flagyl gel for bv but can you smoke marijuana?

Yes. Ethyl alcohol interferes with the breakdown products of flagyl and it is converted to formaldehyde which slowly embalms your liver!
Possibly; need data. It seems unlikely that cannabis or its byproducts could interfere with or trigger side effects to metronidazole use (Flagyl). The risks are probably unknown and possibly very small. Pharmacological studies and more reliable scientific data will be needed to answer your question more accurately.