Desencitizing from known Allergies?

I am 37. I have found that I have developed an allergy to Codeine, Caffeine, Pain Killers such as Panodol, And some foods.
Codeine being the Highest on the list to avoid at all cost! while caffeine makes me feel like I have a bad case of the flue which passes in an hour or so. Pain killers, even in the smallest dose. Makes me feel like I've ingested a Bag of Sugar and my mind is Racing at a hundred miles an hour. I find it hard to sit still, feel uneasy, and need to keep my mind active till the symptoms pass. Which thankfully doesn't take that long.
Codeine on the other hand is a living Hell! I discovered This the hard way by drinking from a glass that was used by my partner who used the glass to mix her Crushed Codeine tablets with water.
I never drank the same water or its contents. Only the glass with a fresh fill of water! The outcome seen me Bed ridden for 5 Days. For the first 3 days I could not drink or eat any thing. I slept for no less the 12 to 15 hours a day. And while I was awake, I felt like I was living in a Nightmare with in my own mind!
after 4 days i thought I was going to loose my mind, and forced my self to see my local general GP, Whom simple told me to "Ride it out" and avoid Codeine. To end the matter, After the allergy attack I also suffered Depression!
Not content. I went to a second practitioner. Whom told me to try a drug called Phenogen. After what I had dealt with, I was hesitant in what effects this drug would have? I decided NOT  to take a whole tablet but break it down into quarters as a trial. It gave me the same reaction as the Pain killers (mentally), But with heavy flu like symptoms, Which took a good part of the entire next day to over come.

Now I understand that I can get my self Desensitized. What I would like to know is What would be the procedure? What I fear most is, that. I will be used as a Hit or Miss exercise while my Allergy is overcome. I have no problem with Flu like symptoms. But the Mental torment I'm not!