Banning recreational drug use certainly reduces the freedom of the user, who is no longer free to do what previously they did. They are no longer exempt from interference. What they previously did voluntarily - use recreational drugs - they are now forced not to do. There is a legal barrier between them and what they want to do. (I assume the law is effective - a large assumption.) This diminishes the area of freedom.
I use 'freedom' here as it is widely used in ordinary discourse. I say nothing metaphysical about freedom of the will.
AUTONOMY - THE DOWNSIDE
How is autonomy affected if it is different from freedom ?
X (a) cannot choose to live in the way they see fit (b) in accordance with the values they have accepted, and (c) carrying out decisions they have made without external hindrance. These conditions of autonomy are defeated.
There are wider and different views of autonomy but in this minimal sense, a ban undermines autonomy to the extent that it blocks any or all of these conditions.
AUTONOMY : THE UPSIDE
However, one can expand the idea of autonomy beyond this minimal view. Suppose, for instance, we take a view of autonomy along these lines :
A person can be said to be autonomous to the extent that (d) what they think or do cannot be explained without reference to their own activity of mind and (e) that activity is competent. Much is vague and contestable here but I have in mind that a person lacks autonomy to the extent that they are unable to conceive a range of alternatives rather than just a single action, to estimate or foresee the consequences of their actions, and to revise their choices on reflection and to act on those choices.
We can add to this provisional account that an autonomous person has the minimal rationality of (f) believing propositions that are strongly supported by the evidence available to them, (g) refraining from believing propositions that are improbable on the available evidence, and (h) striving for a broad consistency among their beliefs.
Now, it would be reasonably argued that in banning recreational drug use, the law (to the extent that it is effective) acts as a barrier to the destruction or impairment of autonomy, as conceived in this way, that can result from recreational drug use. In this sense the law reinforces and protects autonomy.
Gerald Dworkin, The Theory and Practice of Autonomy, Cambridge : CUP, 1988.
Peter H. Schuck, 'Rethinking Informed Consent', The Yale Law Journal, Vol. 103, No. 4 (Jan., 1994), pp. 899-959. (Includes discussion of autonomy.)
R. S. Downie and Elizabeth Telfer, 'Autonomy', Philosophy
Vol. 46, No. 178 (Oct., 1971), pp. 293-301.
Rüdiger Bittner, 'Autonomy Modest', Erkenntnis (1975-), Vol. 79, Supplement 7: WHAT MAY WE BELIEVE? WHAT
OUGHT WE TO DO? Keynote Papers of the Eighth Congress of the Gesellschaft für
Analytische Philosophie at Konstanz, September 17–20, 2012 (2014), pp. 1329-1339.
Thomas Kelly, 'Epistemic Rationality as Instrumental Rationality: A Critique', Philosophy and Phenomenological Research, Vol. 66, No. 3 (May, 2003), pp. 612-640.