Best thing to do is check with your dr. or your pharmacist. My hubby is on 3 BP meds. and both his dr. and our pharmacist recommended Robotussin CF for him and it seems to be fine. There are many OTC stuff, but it might not be safe to take, so check first. Good luck.
I have seasonal allergies, and my doctor prescribed regular ALLEGRA. He tells me hypertensive patients should stay away from ALLEGRA-D ( "D" meaning decongestant), or any OTC or prescription meds that have the decongestant pseudoephedrine in them.
Youu can take any of the antihistamines (chlorpheniramine maleate, diphenhydramine (Benadryl)), cough suppressants like codeine or DM; just not anything containing pseudoephedrine (or any decongestant,) as Zuzu said.
I drink lots of hot tea and raw garlic cloves, works *GREAT* for me, I do not get sick if I take my daily clove during the colder months. I used to not be a big fan of spicy food, but I got used to it since not getting sick was so fantastic. If I slack on the garlic and do start to feel a little sick, I just take a couple cloves that day and make sure to drink a lot of tea and usually feel fine again in no time. I've been doing this for years.