If you do decide to implement drug testing...
Okay, lets assume for a moment that you do implement full "random" testing.
You should (actually, MUST) have an outside firm do this. Pharmacy purchased tests might be okay for home use, but will not stand up for any formal criminal, or civil action.
Okay, so you've checked out the legal history of the firm you've contracted with and, so far, all their appearences in court have 'stood up,' i.e., their proceedures and testimony are fully validated.
Everyone in your firm must be included in testing. All employees, the stock boy, the two forklift operators, the secretary and bookkeeper, and YOU!
Yes, You. When the testing firm shows up, you must be the first one required to pee in the speciman cup, and the troops should see you standing in the same line, filling out the same paperwork, as Mr. Junior Nobody.
Also insure your employee manual states that drug testing is required for all; line employees, supervisors, admin. staff. Also, I suggest putting up signs both in the break room, and at each entrance, that reads "Drug-free Workplace." And where job applicants sit to fill out any pre-employment paperwork (i.e., before the initial interview) another sign that reads "Drug Testing is Performed before any Hiring Decision, and during the work year."
Be sure whatever wording you choose has been reviewed by your company lawyer to insure it meets your local laws/standards.
One key to making this work is if you are seeking any kind of work from Government. Usually the government requires certain administrative rules. For example, under Federal Acquisition Regulations the U.S. Government forbids firms from using prisoners (ex-felons are okay, its those that are on work-release, i.e., "active" prisoners that are prohibited.) So your lawyer should advise you on what local laws/regulations will pertain to the whole process.
Then, once you've a detailed list of requirements, have your accountant determine how much your cashflow will be impacted for initial startup, then follow-on costs.
And as others above have mentioned, only then can you make a sound determination if you should/will implement pre-hire and on-going random drug testing.
One final note... if/when you start searching for a drug testing firm, be sure to ask for a breakdown of what the various tests they can do, and what the cost is. If all you are interested in is grass/weed, that will be one price. If you want to add meth. ("Speed"), that is an additional cost. Toss in coke and smack and now you are reaching for some real cash outlay.
Stan Db