Need to pass a drug test?

I'm a mechanic for a transportation company and our company routinely gives us random drug test because we are sub-contracted through the metropolitan transit system so we need to meet certain federal guidelines to work on their fleet of buses. I personally happen to take daily medicines to maintain my blood pressure, cholesterol, and thyroid levels and I always get nervous when I'm sent to take a drug test. A computer randomly selects the social security number each mechanic has on file. I always wonder if the medicines I'm taking will have any effect on whether or not I can pass drug test. Some people have told me that it can and others have told me that it can't. I shouldn't have anything to fear about because I don't use drugs other than the ones that my cardiologist prescribe to me. Of course when your younger, you are curious and try some non-prescribed drugs but that was a long time ago. I wasn't aware of it but nowadays there are companies that manufacture products that you can take to beat a drug test. And they guarantee it or your money back. So if you really need to pass a drug test but don't have a lot of time, you could try one their products. Sounds pretty simple to me.