Can I use regular potting soil for cacti or do I really need cactus mix?

Cacti evolved in dry climates where water drains almost immediately, so standard potting soil really does invite root rot - I learned this the hard way when I stuck a couple of succulents in regular mix thinking it'd be fine, and within a few weeks they got mushy and died. The drainage difference is huge; cactus mixes typically have sand, perlite, or pumice mixed in so water runs right through instead of pooling around the roots. You can get away with regular soil if you amend it heavily with coarse sand or perlite (like a 1:1 ratio), but honestly at that point you're basically making cactus mix yourself anyway, so picking up a bag of the real thing saves the effort.

I tried it once with a little barrel cactus in straight potting soil and watched it turn to mush within weeks - the roots just couldn't handle how wet it stayed! You really do need that drainage, whether you grab actual cactus mix or just throw some perlite and sand into regular soil to make it grittier.

You can technically use regular potting soil, but you'll be fighting an uphill battle - cacti roots genuinely struggle when the medium stays damp for more than a day or two. If you're set on using what you have on hand, just amend it heavily with perlite, coarse sand, or even small gravel (aim for maybe half the volume being drainage material) to mimic how fast water moves through actual cactus soil. That said, cactus mixes are cheap enough that it's honestly easier to grab a bag and skip the DIY gamble, especially if you're newer to growing them.

The easiest fix is just mixing regular potting soil 50/50 with perlite or coarse sand - that's what I do with my collection at home and it works brilliantly. I had a few cacti languish in straight store-bought mix years back until I started amending it, and the difference was night and day in terms of how the plants actually thrived.