Brief Answer:
it's a real stretch.

Detailed Answer:
The possible adverse events depends a bit on why you are using prometrium.
But, prometrium is almost ready to be thrown out of approval in the US because of risk of embolism (stroke and heart attack).

Marijuana is not a strong risk for either. There may be one or two reports in which you wonder if the marijuana might have caused stroke or heart attack
http://www.ncbi.nlm.nih.gov/pubmed/0000
but in general there is no associated risk of marijuana and embolism, so there shouldn't be an increased risk of it wtih prometrium

OTher interactions do not have evidence for them
basically k, it sounds safe

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Brief Answer:
no overdose

Detailed Answer:
First, they don't interact pharmacologically so you aren't really getting ANY increase in drug nor effect if used together.
Second, there isn't an overdose from the female progesterone drug prometrium like there would be with tylenol, aspirin, heroin, blood thinners, blood pressure medications. A much higher dose of prometrium doesn't do all that much different of an effect than the correct dose. You get maybe a slightly higher but still low risk of thromboembolism, but just as likely the risk stays up but doesn't change wtih more prometrium (the risk is small). You don't get different drug effects with an overdose and its toxicity is small.

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