BodyButter
Vital Football Legend
There's the scope to consider as well, what drugs would be legalised? Marijuana is too easy an example, it is relatively begin for adults (not for young people whose brains have not fully developed though).
Cocaine, ecstasy, other opioids can be extremely dangerous, and even worse in interaction with other drugs (including alcohol).
I think the calculus is somewhat simple, baring in mind, I agree with legalisation. You'd have to accept more people using and abusing drugs and therefore accept more people going to hospital (e.g. for sustaining injuries whilst high) and requiring long term treatment for addictions (you'd have to assume that governments are prepared to allocate increased budgets for this but have to accept this may not happen sufficiently). You'd likely have to accept more drug influenced crime since drugs like cocaine and ecstasy can precipitate violent and/or risky behaviour. You probably wouldn't lose the illegal drug trade entirely.
Benefits are of course listed in other posts already. But again, just keen to stress it would not be a free lunch and could have unintended consequences.
Don't all of those downsides exist at the moment in addition to the violence and gang culture in the UK and the utter destruction of countries like Mexico.