Can you be arrested if you have no drugs on you but you are 'high'?

Actually the existence of drugs in your system is considered possession in most states and you can be charged accordingly. A urine test is commonly requested at the time of arrest if the law enforcement officer has probable cause to believe you are under the influence of drugs such as marijuana. The refusal of a urine test can be used against you as well by the prosecution.

I can't say for the states because I have never been, but in Ireland you cannot be arrested if you have no drugs on you but your high especially with hard drugs because too many people freak out in police custody however if you are found in possession while high even do they will take your drugs you still cannot be arrested at the time but you can be arrested the next day. Also, being high in Ireland is defiantly not worse than possession because possession of even a small amount you can be brought up on charges of possession with intent to supply. You can only get brought up on charges of personal possession and with soft drugs like weed and hash you will generally just get a fine of around 300 euro around 325 us dollars

In Canada if you are impaired you can be charged, be it alcohol or drugs. You cannot be charged with possession only under the influence.

ADDED: In the US you cannot be arrested on a 'narcotics' violation for merely being under their influence. However, depending upon the circumstances at the time of contact with law enforcement, you can be taken into custody on other somewhat un-related charges (public intoxication comes quickly to mind), as both a public nuisance, or for disorderly conduct, or even as a purely protective measure.

ADDED: The definition of intoxication includes impairment of one's normal faculties by ANY substance (including alcohol).

Therefore, even when "high" on drugs, or from 'huffing,' you are intoxicated and can be arrested for public intoxication. See:
Definition of INTOXICATION NOUN: (1) Stupefaction or excitement by the action of a chemical substance. (2) Exhilaration, excitement, or euphoria. (3) Poisoning by a drug or toxic substance.

ADDED: Actually, in the state of California being under the influence of illicit drugs (or prescription drugs if the prescription is not yours) can get you arrested. You would be charged with Health and Safety Code §11550. Yes, you will and can be arrested in California for such offense. However, it is not a crime in California to be under the influence of marijuana, unless it impairs you to the point where you endanger your life or interfere with public right of ways. Please, if you must get stoned, do it in the safety of your residence and don't come out for 48 hours after you have ingested...

ADDED: In most states in the US, it's not "technically" illegal to be high. It's illegal to possess or distribute but if you make the officer upset or are doing something to have attention brought to you, you can get booked for public intoxication and if you're in a vehicle or on a bike you're looking at a DWI/DUI/OVI (depending on the state you're in).