May I esquire as to why you took 9? Was it all at once, or over a specific period of time?

side effects:

fast, pounding, or uneven heartbeat;
weakness, tremors (uncontrolled shaking), or sleep problems (insomnia);
severe restless feeling, hyperactivity;
confusion;
problems with vision; or
urinating less than usual or not at all.

These could be intinsified if you overdose. I would say that you need to get to a doctor if any of these symptoms occur.

@Nicole97 Welcome to Fluther, and please be careful to take no more than the recommended dose of OTC or prescription medications in the future. We’d like to have you around for a good lone while.

Fortunately, Zyrtec (cetirizine HCL) is one of a class of newer antihistamines that are less prone to produce adverse effects with a moderate overdose. If you are not experiencing extreme drowsiness any of the list of overdose effects here, you can probably manage it at home. Stay very well hydrated—lots of water. Flush it out of the system. Don’t do that again.

Were you trying to hurt yourself? If so, it probably means you are depressed or angry or upset or all three. Perhaps things are not going well at home. If you can’t talk to a parent, is there some adult you do trust and can talk to? Even if you recover from this just fine, by yourself, you need to address the reasons you took the pills, or things will get worse. Please find someone you can trust to confide in.

Being a teen is a very difficult thing. Parents and teens often conflict with each other. Teens feel alone and unloved in many cases. Parents don’t seem to understand. Parents make demands that seem too hard and unreasonable. This may be the hardest time of your life. But your goal should be to make it through. Things will get better in a few years. Five or ten at the outside. That may seem like a long time, and you may not even think you’ll make it to twenty, but you can.

Break it down into small problems. Ask us about them. There’s a lot of people here who have been teens at one point and can share stories of how they got through it. People really do care, even if they are strangers.

From medicines.org.uk:

Cetirizine Hydrochloride 10mg Tablets

Toxicity: Limited experience of overdosing. 20 mg to a 2 year old, 30 mg to a 3 year old and 40 mg to an 11 year old did not give any symptoms. 60 mg to a 4 year old gave mild intoxication, 400 mg to a 14 year old gave mild symptoms while 400–500 mg to an adult gave no symptoms at all.

Symptoms of overdosage reported with antihistamine substances: Somnolence, unconsciousness and/or excitation (principally in children). Ataxia, tremor, headache, hallucinations, seizures, dry mouth, flush, hyperthermia, mydriasis, urine retention tachycardia and in the case of massive doses, possible fall in blood pressure and arrhythmias. Nausea and vomiting. Also extrapyramidal symptoms are possible. Cetirizine hydrochloride has a low sedative and anticholinergic effect. Sedation can be a symptom of overdose, it can occur after a single dose of less than 50 mg.

Treatment: At the present time there is no specific antidote. Experience of overdosing is limited and no severe intoxication has been reported to date. Primary treatment should be gastric lavage, if justified and charcoal. Symptomatic treatment should be instituted in the case of acute intoxication, such as diazepam for seizures or acute dystonias.

You are going to have one hell of a case of dry mouth. Drink plenty of water.