They will also be looking for illegal and controlled substances, however this can also be done (and is usually more accurate) by taking a hair sample (even if you shave your head the hair can be taken from other parts of your body).
The presence of an illegal substance in your body is grounds for a rejection of your life insurance application.
In addition to illegal substances they will screen your urine for alcohol, sodium, caffeine, and a number of other substances that will "increase the risk of insuring your life".
Learn more about life insurance drug tests.