Diazepam (Valium) is a long-acting benzodiazepine. The time it remains in the body and the duration of its noticeable effects differ:

• Onset and duration of effects: When taken by mouth, diazepam typically begins to work in about 15–60 minutes, and its noticeable effects can last roughly 4–6 hours for the acute pharmacologic action. In some people, residual effects may be felt longer, especially at higher doses or with existing tolerance.

• How long it stays in the body: Diazepam has a relatively long half-life, commonly cited around 20–80 hours for the parent drug, with active metabolites (notably desmethyldiazepam) that can persist much longer. Because of these long half-lives, the drug and its metabolites can remain detectable in the body for several days to weeks after a single dose, and longer with repeated or high-dose use. Some sources describe detectable presence in urine or blood for days to weeks, and in certain testing scenarios (especially with chronic use) metabolites may persist for weeks.

• Typical clinical use duration: In many guidelines, diazepam is prescribed for short courses, often no longer than 2–4 weeks, to minimize dependence risk. If prescribed longer, gradual dose tapering may be considered when stopping. Individual plans depend on the condition being treated, dose, and duration of use.

Important cautions:

• The exact time to clearance varies by dose, frequency of use, age, liver function, body fat, and other medications.
• Even after the acute effects have worn off, residual drug or metabolites can be detected for extended periods with certain drug tests.
• Do not adjust or discontinue diazepam without medical supervision, especially if it has been used regularly, due to risk of withdrawal and seizures.

If you’re asking for a specific scenario (e.g., duration of effects for a particular dose, or clearance time for a drug test type), share the dose, frequency, and any liver or kidney considerations, and the context (prescribed use, medical condition, or testing).