Not much to worry about I would say, the absorption of diazepam per os is already efficient enough, somewhat of an increase in plasma levels, would merely make it stronger. AFAIK metronidazole doesn't, however, as has been said.
Alcohol, thats another matter, oh boy that wouldn't be pleasant. It inhibits aldehyde dehydrogenase, which normally clears acetaldehyde, the next metabolic breakdown product from alcohol, you can see a relatively mild version of this in some east asian people, who have lower levels of aldehyde dehydrogenase. They get flushed and feel crappy after fairly small quantities of alcohol.
Metronidazole and alcohol with produce a reaction similar to that produced by disulfiram (antabuse), given as an alcohol-aversive treatment to some alcoholics. One drink, and its puking your spleen out, facial flushing, tachycardia, nausea and vomiting. Same happens with certain mushrooms, that are otherwise quite tasty, that contain a compound called coprine. Coprinus is the genus that contains most of the fungi that contain coprine, C.atrementarius, the common ink cap is a decent edible, as long as you catch it before it turns to inky liquified slime, but it you eat it in a meal and have alcohol alongside, then you get that nasty disulfiram-reaction, likewise if you consume some of the ink caps, and drink alcohol up to a few days later.
I came damn close to getting the reaction from hell, a while back, after I was hospitalised with a severe infection. They were about to start an IV of metronidazole, and thanks to the hellacious pain I was in, I had drank most of a liter bottle of southern comfort a few hours beforehand. I told the nurse about to set the IV going, and he said oh no, don't worry, you will be fine.
Sheesh...I had to give the guy a quick lecture in pharmacokinetics and hepatic enzyme pharmacology, while I was half-conscious and in agony. Bloody git should have known that.
Not to mention, after that, he was about to give me a penicillin drip. Just after I told him I was severely allergic to beta-lactam antibiotics. Arsehole told me to wait, after he already started the IV, until he had time to stop it. I actually had to twist and kink the IV line, to stop it getting into me, with the penicillin inches from the butterfly needle in my arm. That was SO close...shit me up big time, to put it mildly.
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