Diazepam (Valium) begins working within 15 to 60 minutes when taken orally, with most people experiencing noticeable effects around 30 to 45 minutes. The speed of onset depends on several factors including how much food is in the stomach (it absorbs faster on an empty stomach), individual metabolism, body weight, age, and liver function.
When administered intravenously, diazepam works almost immediately, within 1 to 5 minutes, which is why this route is used in emergency settings for severe seizures or acute anxiety. Intramuscular injection typically takes 15 to 30 minutes.
The drug reaches peak concentration in the bloodstream around 1 to 2 hours after oral administration. Its effects continue to build during this window, so someone may not feel maximum impact at the 30-minute mark. Diazepam has a long half-life of 30 to 56 hours, meaning it stays in the body for an extended period—sometimes remaining detectable for weeks—even though the initial noticeable effects wear off within several hours.
Factors like age, liver disease, or concurrent medications can significantly slow onset and increase the duration of effects. Older adults typically experience slower onset and longer-lasting effects. Taking diazepam with alcohol or opioids dangerously slows metabolism and intensifies effects.