Medication half life

[Banned]

I've seen the term "half life" used with regards to medication. I know the half life for Cipralex is 27-32 hours. But what, exactly, does that mean? Is that how long the drug stays in your body? Works in your body? Takes to start working?

Can someone please clarify this term?

Thanks.

 

[Into The Light]

it means that if you take say 20 mg, that after 27-32 hours, only 10 mg are still in your system. After another 27-32 hours, 5 mg are left. After another, 2.5 mg, etc. That is assuming you don't take another 20 mg dose.

Do the math and then you'll know how long it takes to be out of your system.

 

[Banned]

Oh ok...thanks ITL. I never would have figured that out.

 

[Daniel E.]

And by "out of your system," I think half-life is in regards to leaving the bloodstream:

Psychiatric Mental Health Nursing - Google Books

 

[Retired]

In addition to the elimination half life, there is another dimension to this aspect of pharmacokinetics, referred to as "steady state".

The elimination half life refers to the time one dose is eliminated from the bloodstream, however, the next dose must be taken into consideration, in a person taking their medication every day.

In a short half life compound, the steady state, the point at which the amount eliminated equals the amount absorbed, is relatively short. In a product with a half life of, say, 8 hours, steady state is reached in two to three days.

The therapeutic benefit of a rapid steady state is the patient can determine how well the dose is tolerated, and the physician can determine if any adjustments are required.

These parameters do not relate to onset of action or relief of symptoms, which may be shorter or longer, depending on the characteristics of the medication. As we know, SSRI's can take several weeks before symptoms might be relieved.

Getting back to steady state, a long acting (long half life) compound which might have a half life of 3 to 5 days, may take a couple of weeks before steady state is reached. In fact, long acting compounds accumulate in the bloodstream, and often contribute to unwanted side effects, because the actual amount circulating in the system exceeds the expected daily dose.

To make matters even more interesting, when a medication is stopped (withdrawn) steady state works in reverse.

Blood levels of a short acting compound will drop rapidly over two or three days, and in medications like benzodiazepines and antidepressants we know sudden withdrawl can cause withdrawl symptoms and a/or a rebound of the symptoms the medication was treating.

That's why physicians require patients to taper the dose over a couple of weeks at least to prevent a rapid drop in blood levels and potential withdrawl symptoms.

There is less likelihood of withdrawl symptoms with long acting medications, so tapering is seldom recommended.

It must be stated rapid steady state is a desirable feature in many medications and physicians who understand pharmacokinetics usualy prefer using a medication that exhibits this property because of its therapeutic advantage over a long acting medication that can accumulate, sometimes unpredictably, over time.

A short acting medication is easier to manage particularly when other (concomitant) medications are used to treat a condition.

Physicians will also take into consideration the manner in which a medication is metabolized, which when added to steady state and its metabolic half life can determine how the medication will perform in the presence of other concomitant medications.

So one particular metabolic property of a medicinal compound cannot be considered in isolation, but rather in the overall picture of the patient, their medical history, their age, and what concomitant medications the patient might be taking.