Phenylephrine: MAOIs. Enhance effects of anticholinergic drugs eg, TCAs. Increase possibility of arrhythmias in digitalised patients. Enhance CV effects of other decongestants. Vasodilators, β-blockers or enzyme inducers eg, alcohol. Cetirizine: May enhance effects of barbiturates, hypnotics, opioid analgesics, anxiolytic sedatives, MAOIs, atropine. Can also mask symptoms of ototoxicity in aminoglycoside antibiotics eg, gentamicin, amikacin, tobramycin, neomycin, & streptomycin. Other medicines for cough, cold or allergy.