An innocent item can turn into drug paraphernalia, depending on its use. For instance, I have dozens of spoons in my kitchen, which are perfectly legal. If one of them is blackened from flame and has heroin residue in it, there would be a good argument that it was illegal drug paraphernalia used to prepare heroin for injection.
In order to convict you of Heath & Safety Code 11364, possession of drug paraphernalia, they would have to prove you possessed the tube with the intent to use it as a "device, contrivance, instrument, or paraphernalia used for unlawfully injecting" a controlled substance.
Are you talking about a length of hose? Rubber or plastic tubing is commonly used to "tie off" someone's arm to make the veins protrude, making it easier to insert a hypodermic needle -- whether that's to legally draw blood or insert and IV line, or to inject illegal drugs intravenously.
Whether or not you can be convicted depends on the circumstances of the case. Like the spoons in my kitchen, you may have innocently possessed the plastic tube. However, if other evidence -- like track marks, from previous injections -- shows the tube was used to inject drugs, you might have a different result.
Please understand that this is a general discussion of legal principles by a California lawyer and does not create an attorney/client relationship. It's impossible to give detailed, accurate advice based on a few sentences on a website (and you shouldn't provide too much specific information about your legal matter on a public forum like this site, anyway). You should always seek advice from an attorney licensed in your jurisdiction who can give you an informed opinion after reviewing all of the relevant information.
It is unlawful to possess any device, contrivance, instrument, or paraphernalia used for unlawfully injecting or smoking specific controlled substances. Possession of drug paraphernalia is covered by Health and Safety Code Section 11364.
NOTE: It is legal to possess paraphernalia until it is used or intent to use is shown. To show specific intent to use, there must be the presence of evidence of prior use (e.g., residue).
Disclaimer: This answer is provided as a public service and as a general response to a general question, it is not meant, and should not be relied upon as specific legal advice, nor does it create an attorney-client relationship.
To add one bit of information to Mr. Marshall's answer:
He mentioned that Health & Safety Code section 11364 makes it illegal to possess things used "for unlawfully injecting" a controlled substance. The language of that section reads:
"It is unlawful to possess an opium pipe or any device,
contrivance, instrument, or paraphernalia used for unlawfully
injecting or smoking a controlled substance . . ."
If that plastic tube (and again, I don't know what type of tube we're talking about - a rubber tubing like a hose or a plastic tube like the barrel of a ballpoint pen or a straw) was used to smoke a controlled substance (i.e. smoking heroin), then it could fall under this section.