It is a while since I translated this but I am pretty sure the term you want is "controlled." As soon as the patient's condition is controlled (i.e. by medication) the greater the chance will be that he will be seizure -free.

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Note added at 58 mins (2010-10-14 16:47:30 GMT)
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Cf. beginning of second paragraph in this article. In the treatment of epilepsy the most commonly used word is controlled. http://en.wikipedia.org/wiki/Epilepsy
Stabilised may also be used but I worked in this field for 20 years and controlled is the general term in the UK particularly when referring to control by medication.

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Note added at 20 hrs (2010-10-15 12:20:52 GMT)
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I was not at my best yesterday evening when I answered this question but controlled can also be used in this sense of diabetics - a controlled diabetic still suffers from diabetes but his/her symptoms are well controlled by medication. It is even possible to speak in psychiatry of person's schizophrenia being controlled. They still have schizophrenia but the frequency of their episodes of paranoia, hallucinations etc is reduced by the medication.

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Note added at 1 day17 mins (2010-10-15 16:05:41 GMT)
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Yes, sorry. I wrote that in a rush and have had lots of other things to do today. It is far better to talk of the epilepsy or diabetes being controlled which is how I phrased it in my original answer. Informally nurses and other carers sometimes talk of a "controlled diabetic" but that is not good! Controlled should refer to the disease or condition; the control is achieved by medication.