The minute you join a national team you realise you have to start ensuring everything you consume is permitted as per the rules, so for me that happened when I was around 15. I had never used supplements when I was younger, anyway.
Since that point I stopped taking medications when I have been ill which have pseudoephedrine in them, and there are also cold and flu tablets which you have to be cautious of. The Wada website has the list of banned substances, so it is easy to check.
When I order supplements, I get them from a trusted company where the products have been batch tested. This is to guard against cross-contamination, as some products can contain traces of steroids if the company also produce them.
My coach, Chris Tidey from Hamilton Aquatics in Dubai, has told me that British swimming has guidelines in place where coaches cannot even tell a swimmer to take vitamin C, for example, as the risk of cross-contamination is too high.
When we travel abroad for competition, we only stay in hotels recommended by Fina, swimming's governing body, because of the risk of contaminated meat.
If you are a registered athlete, every three months you have to give your "whereabouts" requirements. You write down where you will be every day for sure: every training session you will attend, when you are going to be at home, where you live.
You give a one-hour gap when you are definitely going to be somewhere. For me I usually say at home between 10pm and 11pm, as I know I am going to be there.
There is a Wada representative in every country, so it is not an issue that I am not in Serbia. I was tested at a training session in Dubai two months before the Olympics. The testers are good people. When you are supposed to provide the sample, they make it easy for you and explain the process. They make it as comfortable for you as possible. You get to pick your own box, make sure the label is correct, the serial numbers for the bottles are the same, so that nothing untoward happens from outside.
I have had to do four urine tests and one blood test so far, so I know the procedure quite well, but they still guide you through it to make sure everything is in order and you do not mess it up.
Velimir Stjepanovic, 19, is a UAE-born swimmer who represented Serbia at the London Olympics, where he finished sixth in the 200m butterfly final. He spoke with Paul Radley.