Understanding Half-Life of Medicines With the Latest SSRI Anti-Depressant as an Example

A MEDICINE's half-life is the time it takes for its concentration in the body to be reduced by half.
To illustrate, let's consider Escitalopram Oxalate, the third generation SSRI drug used for the treatment of depression and generalized anxiety disorders and sold under the brand name 'Lexapro' in the United States.
A common dose is 10mg a day taken at one time.
Like any intake, Lexapro too goes through the metabolic process in the body.
By the time the 10mg of ittaken is reduced to 5mg, it is 27-32 hours since the intake.
Therefore, the half-time of Lexapro is 27-32 hours.
In some people, however, it may be a day or two longer because of marginally active metabolites (byproducts of metabolism).
How half-life is determined: Half-life is usually determined in relation to the amount of the drug in blood plasma.
The time taken for a drug to be removed from plasma is called 'clearance' and its distribution in the various body tissues is known as 'volume of distribution'.
These two pharmacokinetic parameters are important in determining the half-life of a drug.
Why half-time is important: No matter what pharmaceutical companies may claim, there is no drug that has no side-effect.
Even the simple analgesic, paracetamol, can cause discernible side-effects if taken in excess of dosage.
The side-effects of aspirin are well-known.
If this is the case with relatively simple drugs, one can only imagine the situation with SSRI drugs which tinker with the flow of a neurotransmitter, serotonin, within brain cells.
Given the reality of side-effects, medical science needs to establish how long specific drugs linger in the body.
Knowledge of half-life comes in handy here.
If at one half-life, the concentration of a drug is halved, two half-lives will reduce it to a quarter, and so on.
This enables researchers to calculate, with reasonable accuracy, the time it would take the medicine to be out of the body.
Knowledge of half-life enables doctors to prescribe a discontinuation process, which is very important for drugs because of heir potentially severe withdrawal symptoms.
Given Lexapro's half-life of 27-32 hours, its discontinuation, when necessary, is usually done by reducing dosage by 5mg a day every six-to-eight days.