Duration of Action of Intramuscular Thorazine (Chlorpromazine)
IM chlorpromazine has a duration of action lasting up to 12 hours after a single injection, though the plasma half-life is only a few hours. 1
Pharmacokinetic Profile
The duration of IM chlorpromazine in your system operates on two distinct timelines:
Immediate Clinical Effect (4-12 Hours)
- The FDA label explicitly states that duration of activity following intramuscular administration may last up to 12 hours 1
- This therapeutic window allows for dosing intervals of every 3-4 hours for nausea/vomiting, or every 4-6 hours for severe psychotic symptoms 1
- Peak clinical effects typically occur within 30-60 minutes after IM injection 1
Plasma Clearance (Days)
- The plasma half-life of chlorpromazine ranges from a few to several hours, meaning most drug and metabolites clear from plasma within a few days after discontinuation 2
- Blood studies consistently show half-life of disappearance in the range of "a few to several hours" 2
- Under steady-state conditions, 43-63% of a daily therapeutic dose can be recovered in urine within 24 hours 2
Extended Tissue Persistence (Weeks)
- After drug discontinuation, urinary drug and/or metabolites persist from approximately 3 to 18 days, with sometimes minimal or trace amounts beyond this period 2
- Chlorpromazine is highly lipophilic and accumulates in adipose tissue and other body stores during chronic administration 2
- However, the pharmacological (therapeutic) action of chlorpromazine lasts no more than a few days after drug discontinuation, despite tissue stores 2
Clinical Implications for Single IM Dose
For a single IM injection in an adult:
- Expect therapeutic effects for 4-12 hours 1
- Redosing can occur as early as 1 hour if no hypotension develops (for acute agitation), or every 3-4 hours for sustained symptom control 1
- The drug will be substantially cleared from plasma within 24-48 hours after a single dose 2
Important Caveats
The distinction between drug presence and therapeutic effect is critical: clinical improvement may persist for weeks or months in some patients even after the drug is no longer pharmacologically active in the body 2. This represents sustained clinical response rather than ongoing drug action 2.
For chronic users, tissue stores can result in detectable metabolites for 2-3 weeks, but this does not equate to ongoing therapeutic or adverse effects 2.