HanDor
Q: My 15 yo son just began straterra/atomoxetine. How long has it taken for your body to get used to the medication? He is complaining he's so tired.
Chestnut Hill CAP Elijah is correct about the prevalence of poor-metabolizers in the population (primarily due to deficits in CYP2D6, a hepatic enzyme that catalyzes the metabolism of this sort of drug). But it's uncommon to undergo gene testing to direct management once a med is started because of the time it takes and the weak evidence of meaningful differences in outcome.
However, just because one is a poor metabolizer doesn't necessarily mean they're a poor responder -- it just might take longer to reach steady-state and ultimately show a response. Typically, steady state is reached in several days with this medication and therapeutic response starts after a couple weeks and reaches a maximum in 4-6 weeks.
Strattera is usually given in the morning, but it can be taken at night if it causes daytime fatigue and does not interfere with sleep when taken later. Unlike stimulant medications, this medication works through a longer-term mechanism (though some patients report a short-term "immediate" response as well), so giving it consistently at night should have the same long-term response in terms of controlling ADHD symptoms.
To why your child might be experiencing fatigue right now... this medication (and many oral medications) undergoes what is called first-pass metabolism. This greatly decreases the amount of native compound that enters circulation. Of course, in people with decreased or variant CYP2D6 this amount can be much higher than with typical patients.
It is actually usual for the liver to upregulate production of enzymes when there is a demand, so it might be very possible that your child will adapt very well to this medication given some time. How long? Well, it's variable but on the order of 1-2 weeks at least.
Hope this helps!