Anti doping I believe means any external chemical drug you take to enhahce your performance in sports. In apnea is quite diferrent as caffeine and psedoephedrine increases heart rate and consequently reduces your ability to hold your breath.
There are also other drugs that may increase your performance, but in freediving it won't prove so beneficial. Examples are EPO, steroids like: deca durabolin, primoteston depot, equipoise, primobolan, clenbuterol etc... This are for increasing testosterone levels but in the long run, it will damage your phisique and can give serious and adverse effects, physically and phsychologically.
I believe the only steroid that cannot be detected is HGH or human growth hormone yet(correct me if I'm wrong). In conclusion, in the world of apnea there aren't really no short cuts, and a regular routine with other natural techiniques will prove more beneficial than any other drug in the market.
Roberto Zuniga