You have to pick:

This will seriously mess up your DNA. Also, many enzymes are composed of two or more molecules associated with each other, so this will screw them up badly. Including the pores in your neurons' synapses. Oh, and although your bones are suddenly a lot denser and thicker they wouldn't change shape, so your brain is now 50% larger in a skull cavity that is a bit *smaller* than it was before. I think you'd basically just die instantly (and messily).

This would be instantly lethal.

Firstly, you'd violently implode due to being composed mostly of water that's suddenly at half of its equilibrium density. The water molecules will pull together to restore their equilibrium density, causing you to collapse to half your regular volume.

Your bones probably *won't* collapse, though they may become rather brittle. That makes a messy situation downright gruesome.

If some miracle handwaveyness is used to rescale you down "gently" somehow, you'd still die within a matter of seconds. Every cell has 23 pairs of chromosomes, and as a rough approximation you need all of those present to live.[1] If every molecule has a 50% chance of disappearing, the odds of any given cell keeping all of its chromosomes is 0.5^46, or 1.4*10^-14. That's not good odds, basically all of your cells are going to die.

Also, most enzymes are actually complexes of more than one molecule working together. Hemoglobin consists of four protein subunits, each a separate molecule, with a fifth molecule (Heme) in the center. Remove any one of those molecules and that hemoglobin complex is going to have a bad time holding oxygen properly, and odds are that only 3% of hemoglobin complexes will not lose any of their five molecules. Neurotransmitter receptors are connected to ion channels made of multiple protein subunits. And so forth.

So boom, you die.

*[1] Women can actually be born with only a single X chromosome, it's a condition called Turner Syndrome. Men could lose their Y chromosome without fatal consequences, since that's genetically identical to Turner's. And people with Down's Syndrome or one of several rare supernumery sex chromosome disorders actually have an extra chromosome they could well bear to lose. But since the molecules are being removed randomly none of this is really going to significantly boost your odds of survival.