As far as I'm aware you can ask in a medical declaration whatever like as long as it relevant to the role. Questions like have you ever suffered from epilepsy, allergies, and so on are quite acceptable if it is in the best interests of the employee/employer relationship. Any medical information should of course be confidential and only those who need to know should have access to this information.

We ask lots of questions of our employees to satisfy ourselves that we have selected 'suitable' staff for working at heights.
e.g

Occupational History:
1 Has your employment ever been terminated on the grounds of ill health?
2 Have you worked in an industry with high noise or vibration levels?

Medical History:
1 Do you smoke?
2 How many units of alcohol do you drink per week?
3 Do you have any physical disabilities we should give consideration to?

And so on and so forth...

Our questionnaire was compiled by a health proffesional and satisfies our clients requirments for health monitoring, although we don't provide any OH services either.

All info is reviewed by myself and held secure by HR.