I think the dose depends also on what you are taking it for (epilepsy, depression, bipolar, etc.) and also what other meds you are taking.

http://en.wikipedia.org/wiki/Lamictal
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A therapeutic response may require weeks or months of subsequent dose escalations, and very small differences in dosage often have noticeably different effects, much more so than with most other psychiatric medications; as little as 10% more or less may make a noticeable difference.

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Good luck.