I understand the behavior can't be allowed and otherwise would have a coupla suggestions, but having grown up on many different allergy/asthma meds myself (I'm still on a nasal spray and zyrtec daily) may I address the other issue?
All allergy meds (no matter which brand) affected me physically, mentally, and behaviorally. I've been through nearly everyone ever on the market. They affect everyone a lil differently, but make no mistake: they definitely change one's thought patterns, ability to exhibit self-control, and overall ability to deal with the sensory input received from the rest of the world.
As a child on those meds, I was always grouchy, tired, didn't want anyone to touch me, sensitive to light and extremely sensitive to sound. I couldn't stand to be touched. Your body feels heavy and light, hopped up on caffeine and drugged up on morphine at the same time. It just makes you "prickly", for lack of a better term. And they make you much more aggressive about, not getting what you want, but being left alone. You don't want to be that way, but you are. And as a child, I had absolutely no ability to restrain what I said or did in order to be left alone. Because alone is the only way to stop the extremely irritating sensory input. It's almost like being a raging autistic.
As an adult, zyrtec continues to make me feel this way a bit...but as a child, it was overwhelming, but it was either that or be sick and absolutely miserable all the time.
My son has had a few days he's had to take zyrtec and it just makes him insane - can't stop talking, bouncing (literally) everywhere including off the walls and beds....
Who your son is on the allergy meds is not who he is. It's a synthetic existence that changes one's personality for the worst. If you can take him off any one of the number of meds he's on, or take him off for a season (I got to where I'd only take meds on my worst days in the spring and fall), try it. If he can have any significant time free from the meds (usually not possible, tho), I'm sure you'd see a significant difference in him and your relationship with one another could heal a bit.
In regard to the hitting, yeah you're right - it can't continue. It is, however, normal for this age. (Most children I've known either hit or bite...my son did both, lucky me). My suggestion: let him know that hitting is wrong and why. If you're a believer, our family uses the If-Then chart from Doorpost.com...we point to the offense, read what Daddy God has to say about it, explain in no uncertain terms why it's wrong (in this instance - God created us and therefore we are precious to Him. To hit someone who is precious to God is to demonstrate a lack of love and respect for them, and thus for God. We are called to love those around us, especially our families.), then point to the discipline (age and context appropriate - at two we would use a hand slap and isolation until restoration), perform it, then when the child is ready to restore the relationship, they must come, apologize for the offense, ask for forgiveness. We then evaluate what could have been done/said differently for next time and never bring it up again.
I know it's a long process, but it's consistent and the child always knows what's going to happen if he makes the choice to misbehave in some way. And it keeps us accountable to consistency.
Good luck!! Give your little boy a big hug for me and let him know that (altho it may take 20 years) he may eventually grow out of those allergies and asthma (playing a tuba helps...)!!!