[acecharly (OP)]

When you ask a chemist for medicine for certain problems they allways say areyou on any other medication. With millions of different medicines how do they know what will react with what?

Cheers Ace

[CliffordK]

A pharmacist should be aware of the properties of the medications that he or she is selling.  The doctor should be aware of the drug interactions, but the pharmacist might be a final check before the patient gets the meds.

There are now computer programs to track drug interactions.  If one buys the medications from the same pharmacy, or pharmacy chain, then they should already track what has been purchased.

One can look for drugs with similar mechanisms.  For example, it could be problematic if a patient was to be prescribed multiple antidepressants, at least without taking each other into account.  The same might be true with pain killers and sedatives. 

Or, perhaps a patient would forget that they were supposed to stop taking an old medication and start taking a new variety that did the same thing.

There are, of course, drugs that also interact with the diet such as those that interact with alcohol, or Warfarin/Coumadin users  who must pay attention to vitamin K supplements and dietary sources.