Drugs in sport
It's sad, but all sports are having to become increasingly alert to the dangers of performance-enhancing drugs, and sadder still that there need to be rules for quite young performers. But these things are decided internationally and are completely out of the hands of the British swimming authorities.
No-one is going to be asked to pee in a bottle at club, county or district level - and at the level where testing may be applied, the rules governing procedures are very strict and designed to minimize embarrassment. But anyone reaching age group nationals (for which the youngest category is 11 years old as of the last day of the championships, in late July or early August) may be selected completely at random for a test, and swimmers over 16 who reach the level of the World Class programme may also be tested away from competition at any time.
From press reports, it seems most of the famous/notorious positive drug tests in various sports can be traced to dietary supplements. The theory being either that manufacturing processes may allow accidental contamination or, more sinisterly, that banned substances may be included surreptitiously in the recipe and the athlete taking them duped into thinking s/he is staying within the rules. The advice to parents and swimmers from the ASA's Head of Legal Affairs at the induction day for the 2002-03 World Class Start and Potential programmes was do not take supplements, full stop.
Then there is the matter of everyday medications which may dry up that cold nicely but cause you to fail a test. Fortunately, there is now a guide through this minefield - the drug information database provided online by the government agency UK Sport.
This is searchable by sport (and bear in mind that that other sport you are finding less and less time for now swimming is going so well may have different rules), governing body (use the ASFGB) and product/brand. It will tell you the active ingredients and whether it is cleared for use - but even then, you should check that the ingredients listed on the product match those recorded by the database. (A British skier was stripped of an Olympic medal for something as simple as using a US-made version of a nasal spray which contained a banned substance not in the product sold in the UK under the same brand name, which was cleared for use.)
Finally, asthma is of course not uncommon among swimmers. The webmaster is not remotely qualified to pronounce on the subject, but this link will take you to the relevant section of the British Swimming web site.

