They are easy to use but do you have a confirmation test process in place and do the employees understand the process as this would  be different form the saliva test (urine usually)

you probably already have the following covered:

it wouldn't hurt to have a support/counciling service in place if you dont already (http://www.acas.org.uk/index.aspx?articleid=4387)

have you though about all the prescription drugs and foods that will give a not negative result and do employees understand that verification of their medication will be required (prescription drugs can be abused too).  How will you handle not negative tested employee while you seek confimation

how would you deal with not consenting to tests

It is good that you have your policy in place, however is your policy sufficient and covers everything?

Few things to consider, apart from the intro, purpose, scope etc.:

-      Legal limits

o    are you going for driving limit with regards to alco or zero tolerance due to the working environment, etc.

-      Illegal substances (what you are testing for)

-      Dependency

o    do you have a programme to help employees if they come to you?

-      Medicines and prescription drugs

o    how they inform you about any changes to medication

o    do you check if medication was prescribed or it is over the counter

-      D&A testing procedure:

o    How do you test (saliva / urine sample / hair / breathalyser etc.)

o    When (after accident, randomly and, or suspicion)

-      Actions following tests results (most important)

o    Are you sending the results to the lab for confirmation (easier to defend a claim if someone was positive due to dietary reasons e.g. poppy seeds shows up as opiates incl. heroin

o    If answer to the above is yes then are you paying someone while awaiting the results?

I went through D&A awareness and testing procedures and been developing policies / procedures within the past few years and believe me, you need a comprehensive policy, good training and to follow the chain of custody as there will be different individuals that can easily claim.

I suggest you attend D&A awareness and testing training. Training costs are around £250(this will equip you with knowledge and necessary forms such as consent forms etc.), also from my experience I would not go with saliva tests. Urine samples + breathalyser are much better and cheaper option. We use alcomate 500 (in range of £500).

Look at some of the pros and cons:

-      Saliva tests quick and easy to carry out – samples vary £5-10 but if you like to carry out confirmation test you will pay up to £300;

-      Urine samples £5-10 – to send the samples to the lab it will cost up to £60 incl. postage – results up to 24h;

-      Saliva only detects drugs up to few days, while urine sample even up to a month (THC);

Please let me know if you need any guidance or help with procedures, equipment or anything else. 

Additionally what are you going to do if an employee refuses

to tell your company what medication they are on? You can’t make them. 

You may have better luck persuading employees if you use an external company.

Also what will you do if you find someone, rehabilitate /sack ? The person in question is your best…Welder, draftsman, manager, gismo

operator, doodad maker or a director? Someone makes a mistake once deserve to lose their job, someone has their food /drink spiked deserve to lose their job, refuses to disclose their medication deserves to lose their job.

Organisations like network rail have a very strict policy on this which extends to their subcontractors.

Or eat too much sesame seed covered bread!

https://www.telegraph.co.uk/news/2017/05/29/angela-rippon-tested-positive-opiates-eating-poppy-seed-bread/

I used to use similar saliva kits and undertake the testing, there is a lot of normal medication that can trigger these. 

I have no issue with using the saliva test, although it doesnt detect as long as urine for the THC, remember the sole purpose for testing is to check if people are working under the influence. 

They are good but only a first indicator, in order to be used properly, you get an indication from the kit and you must send away to the Lab for a full assessment when you get a positive test. 

For alcohol, use a breathalyzer, ensuring it is correctly calibrated. 

I've found in my experience the power of the testing regime is not actually getting the positives on the tests but just the sheer presence of actually carrying them out. 

Also, I would always recommend before you start a program you sit down and work out what you will do with people when a positive is discovered. If you uncover and addiction, it is a health issue and I personally belive employers should offer assistance rather than dismissal in these type of circumstances.