Hi B and welcome to the forum, so sorry for your need to be here. A word of caution, if this is your legal name you may want to change it, this is an anonymous site, but the public does have access. Anonymity allows us to write about some very private things.
I am so sorry about your recent discovery about your fathers drug use. It cuts to the core of us when we learn of a loved ones addiction.
There is a very good article on detachment in the P.E. forum, I have linked it below.
http://www.conductdisorders.com/community/threads/article-on-detachment.53639/
Knowledge is power and a very necessary tool to help us navigate this journey we are on with our loved ones. It is because addiction is a very powerful thing, and we have to be careful that we do not lose ourselves in trying to help our beloveds.
Your father is an adult and will do as he chooses. If he chooses to listen to you and go to rehab, that is great.
Unfortunately, you do have to be ready for the other choice, that he will deny, or admit the problem, but not be ready to quit. No one can force an adult drug user to stop the madness and choose better.
I wish it were different.
One never knows.
Since you have already noted changes in his behavior, the one thing you have to be careful of is your safety, if and when you decide to speak with him. Please make sure you are in a public place, where you can get help if you need to. That may sound outrageous to you, but when our loved ones are using, behavior can be quite unpredictable. Especially if your dad has been using for awhile. His first reaction may be to deny it adamantly, that is my experience with my two.
Be prepared for whatever the reaction is. I think honesty is the best policy. So, telling him that you care for him deeply and are concerned for him is one thing to think about. He may be very upset that you went into his cupboard and found the evidence, had it tested. If it were me, I would proceed very cautiously and try to be as brief as possible. It will probably come as a shock to him, at the very least.
You do have time to sit with your feelings and keep posting here, as well as do some research on the web. Alanon and Naranon groups are helpful. There is much info on the internet. Consider seeing a therapist, it helped me to be able to sit with a professional who had experience and was able to provide me with available resources. For the most part, the concentration was on how I could build myself up to deal with the madness of my adult children's drug use. I encourage you to do the same, the only real control we can have is over ourselves. So please do take some time to strengthen yourself to be ready for whatever the outcome may be.
Stay with us and keep posting. Most of us here are parents dealing with this issue, not the other way around. Really, it is the same, a loved one is a loved one, be it parent or adult child. My heart goes out to you B. Welcome to the forum.
One of the best bits of advice I have received here is to slow....way.....down. When we are confronted with the awful truth of our loved ones drug use it sends our hearts and thoughts racing and it feels like something needs to be done right this second.
In reality we do have time to think things through and sit with our feelings so that we can act with good judgement and prudence.
Take care dear, and stay with us. This is your journey, as we are on ours, but we all go through similar circumstances at different places along the path.
We do our utmost to help one another as best we can.
Take courage and take your time
one step, one day at a time.
(((HUGS)))
leafy