I took a Valium yesterday evening around 8:30. How long will I have to wait before I can take NyQuil to treat my cold?
There is a not an interaction per say between Nyquil and Valium in terms of two drugs interacting with each other, but they do have additive side effects. Nyquil contains an ingredient known as doxylamine which is a very sedating antihistamine. Valium, being a central nervous system depressant, also has significant sedating effects. Taking both together can potentiate the sedating effects of the medications. It usually is not recommended to take both together.
Now you mentioned that you took your last dose of Valium at 8:30 last evening. Valium is an interesting drug in terms of it's effects on the body. Valium is known to have a very long half life (time is takes for 50% of the drug to be metabolized in the body) at around 30-60 hours. This means that the drug would be able to be detected in your body for several days after ingesting a dose! The odd part is that the clinical effects of the drug last much much shorter than the half life would suggest. They typically only last between 8 and 12 hours. If you are not feeling any of the effects of the Valium any longer, you would probably be safe to take the Nyquil as you wouldn't have to worry about the additive side effect of sedation.