The goal of this conversation isn't to win it. It's for them to still want to have it with you the next time something worries them.
A teenager who senses a conversation is a lecture tunes out within ten seconds, even while nodding along. What tends to close things down the most is often exactly what comes from the best intentions: the list of dangers, the warning tone, the comparison to "when I was your age." The goal isn't to convince them you're right — it's that next time something worries them, they still count on you.
Better a calm moment, with no urgency and nothing serious having just happened — never in the heat of an argument or right after finding something out. And better a conversation than an interrogation: ask before stating, listen before opining.
A teenager who feels they can tell you the truth without their world falling apart comes to you sooner when something goes wrong. That is, in practice, the best prevention there is.
Trying something once isn't the same as having a problem. Reacting as if it were — with disproportionate punishment, extreme surveillance — tends to push toward hiding, not toward prevention. If you're worried it's more than a one-off, that's the signal to seek professional guidance, not to manage it alone at home with stricter rules.
If there's a crisis or immediate risk, this is not a substitute for emergency services — call 112 (Spain) or your local emergency number.
Professional guidance gives you concrete tools, not just general advice.