Deionized water is not poisonous and drinking a glass of it will do no harm, which is the honest answer to the direct question. It is also not produced as drinking water, is not disinfected, tastes of nothing, and in some settings has been handled in ways that make it unsuitable. Those distinctions matter more than the toxicity question people usually have in mind.
What the resin removes and what it leaves
Deionization exchanges dissolved ions, both positive and negative, so calcium, magnesium, sodium, chloride, sulfate and nitrate are stripped out. What it does not touch is anything uncharged: bacteria, viruses, particles and many organic molecules pass straight through, because there is nothing for the resin to exchange. That is the important point for drinking. Very low dissolved solids is not the same as safe, and a deionizing cartridge is not a disinfection stage or a mechanical filter.
The resin bed itself is a growth environment
Ion exchange resin sits wet, warm and full of surface area, which is a comfortable place for bacteria to grow, and systems that are used intermittently are the worst case. In laboratory settings this is managed with recirculation, ultraviolet and regular attention. A car washing or window cleaning deionizer in a garage receives none of that. Water from such a system may be excellent for its purpose and entirely unsuitable to drink, and that has nothing to do with the mineral content.
The flat taste and the mineral question
Dissolved minerals give water its taste, and stripping them out leaves it flat, in the same way reverse osmosis water tastes flat before remineralization. On mineral intake, for someone eating normally food supplies the great majority of calcium and magnesium and water a small share, so occasional deionized water is not a nutritional issue. The USGS describes hardness as dissolved calcium and magnesium, classified from soft at 0 to 60 mg/L as calcium carbonate upwards, and deionization removes both entirely.
The practical answer
If it is produced by a system intended for drinking water, with appropriate filtration and disinfection in the train, deionized water is fine to drink and will taste flat. If it comes from a technical deionizer for cleaning, aquariums or a workshop, do not drink it, not because of the missing minerals but because nothing in that system was designed to make water safe. For drinking, reverse osmosis with remineralization is the sensible household choice. Our softener guide covers the hardness alternative.
Questions people ask about can you drink deionized water
Is deionized water harmful to drink?
Not inherently. The concerns are that it is not disinfected, not filtered for organisms, and tastes flat, rather than that it is toxic.
Does deionization remove bacteria?
No. Resins exchange charged ions, so organisms and particles pass through. The resin bed itself can also support bacterial growth.
Why does it taste flat?
Dissolved minerals give water its taste and deionization removes them, the same effect as reverse osmosis before remineralization.
Can I drink water from a car washing deionizer?
No. Nothing in that system is designed to produce safe drinking water, whatever the dissolved solids reading says.