Most hallucinogen users will experience some long-lasting effects, especially to their vision. The nature of it can vary. This can happen in only one use.
If by "precautions" you mean "restricting people's bodily autonomy and sending cops after people who don't comply", as is currently widespread policy, this is very clearly not very effective and either of the widespread direct harms caused by implementing these policies - not the least of which are the chilling effect they've had on basic research as well as a proliferation of erroneous psychopharmacological pseudoscience - and the obvious erosion of the fundamental right to self-determination and bodily autonomy would to me seem more than adequate to outweigh such a benefit even if it weren't a total fiction. Carceral prohibition is bad policy on principle and in practice