opiates and allergy pills?

so i get real bad allergys in summer and ive been taking an allergy pill once a day that lasts 24 hours. today i found a pill on the floor in my house and looked it up...its methadone. what i wanna know is will i have any bad side-effects taking this methadone if i took a 10mg claritan a couple hours ago?
+ rep for help
peace

If it's an antihistamine then it'll cause more drowsiness and might/probably will put you to sleep.

ya its a antihistamine...but if it wil only make me drowzie than i can handle it