The approach should be dependant upon the customers personal situation.
E.g. If they have never been diagnosed with a particular condition, have never been suspected of developing it, or they have never developed symptoms that have prompted the medication. Then I would say that just the medication can be noted, rather than declaring the condition.
If for instance, a customer takes clopidogrel to reduce the risk of a stroke or a TIA, and this was prescribed following a suspected TIA/stroke symptoms etc… then I would still advise to declare the condition that the medication is intended to prevent.
However, if the customer is unsure of why they are taking a medication, as we have no form of medical training and we cannot screen for tablet names or forms of treatment, please refer them back to their GP to find out what condition they are taking the tablet for, even if it is just a "preventative."