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Theme of the Week: Battle of the Longest Half-Lives

A half life is defined as the time for the medication level to reduce by 50%. Long half lives can be beneficial (less withdrawal, more sustained response), but could be a problem if the medication needs to be stopped immediately in the case of emergencies.

Here we are going to explore medications with very long half lives, more than your usual measurement of hours.

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1) Fluoxetine (antidepressant): 4 to 16 days for the active metabolite. Patients who miss a dose are unlikely to go into withdrawal, but when switching to another serotonergic agent, you must be careful because of serotonin syndrome. A 35-day interval is recommended when switching from fluoxetine to an MAO inhibitor for this reason.

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2) Leflunomide (immunosuppressant): 2 weeks. Upon repeated dosing, leflunomide can stay in the body for up to 2 years. Any woman or man looking to conceive will need to undergo an 11-day removal protocol using cholestyramine or charcoal to allow for a faster elimination.

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3) Mefloquine (anti-malarial): 21 days. This makes sense because it can be dosed on a weekly basis for malaria prevention.

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4) Acitretin (acne): a vitamin A derivative which is very teratogenic. When metabolized to etretinate, it has a half life of 120 days. Because of this, female patients must use two forms of contraception before, during and even 3 years after stopping the drug. The concomitant use of alcohol and acitretin can further extend the half-life.

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5) Amiodarone (atrial fibrillation): 60 days. This medication is stored in the tissues and slowly released to control the heart rhythm. The delayed onset of action is the reason why there is a very large loading dose as follows: 800 to 1600 mg daily for 1 to 3 weeks, then 600 to 800 mg daily for 1 month, then a maintenance dose of 200 to 400 mg daily.




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