You CAN use bleach, alcohol, or other such chemicals to "clean" the syringe - is it 100% GARE-ON-TEED safe? No, of course not, but it's better then nothing .. personally, if the only thing available was a used syringe, I would smoke or snort the drug before I would use someone else rigs (even a "cleaned syringe") .. I have shto up a little over 2,000 times, and NEVER did I used a syringe that was used by someone else.. even when I was a meth addict who had pawned most of my shit for drugs, I was always willing to find $2.47 for a pack of 10 syringes..
I have no pitty for those that get some disease from sharing works, because there is no excuse...
But if you feel you MUST, then start by drawing up bleach - fill it all the way... spray it out.. repeat this about 3 times, then use warm water another 3 or 4 times.. I would even go further and do everything over once or twice..
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"I am not one of those weak-spirited, sappy Americans who want to be liked by all the people around them. I don't care if people hate my guts; I assume most of them do. The important question is: 'What are they in a position to do about it?'" (William S. Burroughs)