Certainly there are cough syrups which can push up the sugar levels, but if he is using a sugar free one like coryx, then it is harder to explain why the levels are going up. Potential reasons are if he has a co-existing infection, has been given steroids, or is using something with pseudo-ephedrine. Regarding the painful legs when the sugars are high, this may be due to temporary nerve pains, which settle when the levels come down. Treatment is preventative, i.e making sure that the levels do not go up.
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