In brief: Not exactly. Blood actually doesn't get thinned.  what we call blood thinners are agents that decrease the ability of the blood to clot, usually used in people whose blood clots excessively.
Taking B-complex vitamins, which include vitamin B12, can cause urine to have a bright yellow or even orange color, but check with your doctor to be sure that's what is going on in your case. B vitamins are water-soluble vitamins, which means that what your body doesn't use is excreted in your urine. Riboflavin (vitamin B2) is especially likely to cause this color change in urine. Several medications can also turn urine a bright yellow or orange color. Changes in urine color may also signal certain health problems.
B complex vitamins are water-soluble. Though the body can store some of the B vitamins, they are absorbed, used and then excreeted. Expect urine to be yellow when taking a B complex vitamin.here are several simple things to keep in mind when choosing a B complex vitamin. First, eliminate any brand that has any kind of artificial coloring or dyes. There is absolutely no need for them. For over-the-counter vitamins, I generally prefer capsules over tablets, as many tablets are hard to digest.