Needle/Syringe Because the BTX are not viscous, more like water, they can be injected through a very small diameter needle. Actually insulin needles are frequently used. The needle or syringe (if one piece) should be changed frequently. These very fine needles become dull very quickly, generally after 2-3 injections. Dull needles push, rather than cut, their way through the skin causing more discomfort and increased bruisability.