In terms of protective characteristic, this is not based on the medication someone takes, it is based on the condition. If the condition is more than minor / trivial, has a substantive effect on their day to day activities and has or is expected to last for more than 12 months, it is likely the employee is covered under the provisions of the Equality Act 2010, however only a tribunal can make this judgement, but you are absolutely right to be cautious here.