Hi all!

I've had this cacti for about 5 years now but I've never been able to know what it was.
It has 3 sides and has pairs of thick needles on each of the ridges.
1st year it grew the 3rd and 4th section (got as large as the base) but since then it's been growing taller and taller without the sections getting large at all. Figured it was due to having tall windows.

I'm at a point where I'll need to prune a part of it to prevent it from breaking but I'm not too sure how to proceed.

Thank for your help!

Hi, welcome!

That is a nice plant that is not officially a cactus. It is a Euphorbia. I hope someone else with chime in with a species name as I cannot think of it.
Your plant is suffering from the low light conditions it has been growing in. You can see that from the thinner less solid and more stretched look the new growth at the top has compared to the more robust growth at the base. These plants really want to see a lot of direct sunlight - especially if they are behind glass - they want to see the sun as much as possible.

If you do prune, be careful. Euphorbias have milky white latex-like sap that is both toxic and caustic - so make sure to wear gloves, protect your bare skin and especially your face. Do it somewhere where if the sap spills or gushes out (which it does in some) it is not a big deal. I'd prune right above the last thick more 'normal' looking section. If you improve the plants light conditions at the same time, new growth might end up more like the base.

Good advice.

Cow's Horn (Euphorbia grandicornis)

Thank you very much for the replies!

Happy to finally know what it is and the advices will be pretty useful (and important regarding the sap!).
Will be moving within 2 months so I should be able to get it the sun exposure it deserves

Spraying the cut with water should help with restricting euphorbic sap from continuing to flow. I believe this is what I did when I was propagating E. trigona a lot that worked well for preventing a sticky sap mess. It's probably been about 5yrs, and I'm a little hazy about that long ago, but this feels like how I managed that, I believe my original greenhouse teacher taught me this.