They wont be able to smell it if it is in your system. And even if they did a police search would show you clear of any drugs. It is illegal to posses and sell drugs but they cant do anything if you have already taken them, unless you are for example driving.
If you had been smokibng 'crack' the sniffer dogs might be able to smell it on your clothing.
Actually yes they can, if you have snorted it or smoked it then residue would be detectable by a sniffer dog for quite some time afterwards, you may not think that there is anything left but a dog's sense of smell is said to
be a thousand times more sensitive than that of a humans,
In fact, a dog has more than 220 million olfactory receptors in its nose, while humans have only 5 million. In case you are wondering how I know, my hubby has trained sniffer dogs in the army.
Point of interest, in the paper today a sniffer dog at HR airport can differentiate between banknotes of different countries, also if someone has it in a bodily cavity (for want of a better word) he can still detect it!