alone on a simple traffic stop would not constitute probable cause to search. Are they insulin needles
used by heroin users? Are you driving through a high crime/narcotic neighborhood? Do you have actual narcotics in plain view?
While it may be reasonable suspicion, it is not PC enough to go through your vehicle. Being in possession of syringes
in Texas is not against the law, with or without a receipt. In order for it be paraphernalia, there has to be something else to go along with it. For years we used to use the fact that someone who had zig zag papers on them was in possession of paraphernalia, or even a small glass pipe, however, if you just bought the pipe at a shop and there is no residue or you aren't in possession of MJ, it is not paraphernalia, same holds true for zig zags....many people roll their own cigarettes. See what I'm saying?
It doesn't mean the officer won't ask you for consent to search your vehicle, but being in possession of one item without evidence of another isn't quite enough PC to automatically go through your vehicle. Understand that in Texas, you can be arrested for any traffic offense, except for two, and those are speeding and open container. Should the LEO really want to search your vehicle, or, pardon me, Inventory your vehicle, after you've been arrested, it's gonna happen.
If you buy them in a pharmacy, keep them in the bag, and stop worrying.