Target Dosages
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Q:  Target Dosages

Dr Phelps,
I was reading something recently on one of your archives where you mentioned a "target" dosage for Lamictal at around 200mg. Does the "target" dosage for a med not depend on the size or weight of the person?  Also, is there such a thing as a higher tolerance for medication in general in an individual, despite that person's size?  In other words, could a female patient weighing 130 pounds have a higher tolerance than a man of a bigger stature? 


Dear Ms. L' -- 
Understandable question.  It is actually pretty amazing that we don't see more of an effect from body size, such as you are wondering about.  But it seems that the natural variations in liver metabolic rate are quite a bit larger than body size, for most medications.  There is a known broad variation in the activity rates for the enzymes in the liver that remove medications from the bloodstream.  If someone gets a slow set of these enzymes, they will have much higher blood levels of a medication for a given dose than someone who gets a faster set of them.  The jargon term in medical school for this phenomenon is "slow metabolizers" versus "rapid metabolizers".  

For some medications there really is quite an effect of body size.  Depakote is one of these.  I've not heard people remark about this for lamotrigine though.  

So, you're right, it's really possible for a woman of 130 to end up taking more medication than a much bigger guy to get the same blood level of a medication. 

Dr. Phelps

Published September, 2004


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