It all started when John was offered a certain substance at a party.

You can use the program to create a personal history of problems that are felt to be related to certain substances.

These plants become rich in a certain substance and can then be cut down.

Certain other substances are sometimes also considered to be dust.

Being exposed to certain substances at work or in the environment.

These tests measure levels of certain substances in the blood.

Those who suffer from the disease can not make certain substances in the blood.

It works by changing the activity of certain natural substances in the brain.

Government has made the sale and use of certain substances illegal.

The intention was to see what effect certain substances might have on him, but the experiment did not meet with success.