You can find find lot of information about why you should not take decongestants and Antihistamine Tablets. As the science makes further progress all these medicines are being banned by government sponsored agencies.
Decongestants cause dehydration by stimulating specific systems of the body. Same is done by Antihistamine tablets. But people who developed these tablets had limited knowledge about the human body, so they could not find a reasonable cure of sinus, cold and cough.
All these medicines have serious side effects. Decongestants cause high Blood pressure. All the stimulants will cause that. By stimulation dehydration is done.
Antihistamine also helps in dehydration. But neither of the dehydration systems targeted by antihistamines and decongestants are perfect and hence causes serious side effects.
Slow and steady dehydration of the body by restricting fluid intake is perfect cure of sinus and cold and cough. read earlier posts on how to achieve desired dehydration to cure sinus.