Ok so my girlifriend took 60ml of zzzquil at once will that cause any harm?

Antihistamine od: She should be seen at the ER- she will soon fall into a very deep sleep and will need IV hydration to remove medication from her system, not to mention any possible neurological damage.

Diphenhydramine: That is equivalent to 100mg of diphenhydramine. It is a high dose of antihistamine but standard for some allergic reactions and sometimes motion sickness and unlikely to cause harm in an otherwise healthy person who is not taking other drugs or consuming alcohol concurrently. Something to know is that diphenhydramine is not effective as a sleep aid (despite misleading advertisements).

Maybe: You have to be very careful when taking sedating drugs even if they are over the counter. You should always take only the recommended dose unless directed otherwise by a physician and you should never mix this type of drug with alcohol or other sedating medications or drugs. Most likely she will be ok and may just feel a bit hung over this AM, but if you cannot wake her up take her to ER.