I was reading through the long-form drug details on Lamictal because I was interested in the pharmacokinetics - half-life and relation of body weight to dosage.

One thing that jumped out for me was that it said that for bipolar, there was no substantial/consistent improvement for 400mg vs 200mg. The conclusion was basically that if 200mg isn't doing it, 400mg probably won't either (and side effects become more likely).

So the add-on drug is probably the way to go, as others and your pdoc have suggested...