Can you prevent b5 in Sicilian Najdorf with a4?

Or is there another way to prevent it?

In the Najdorf main line, after 5...a6, playing a4 is pretty much the only way to prevent b5. It is relatively slow compared to other continuations by white in the Najdorf, but it is definitely playable if you want a slower, less sharp alternative to the usual Be3, f3, Qd2, etc. with opposite sides castling. If you look in the opening explorer, 6. a4 has been played over a 1000 times by FIDE masters with good results for white, so it is certainly a respectable move.

@Serial_Checkmater the thing is this pawn on a4 is a little bit awkward and a pawn chain isn't possible because of the semi-open c-file... but yes I would like to slow things down on the queenside, I'll try it out

6 a4 is a bit early, but 6 Be2 intending a later a4 is good. Example:

Here's another game with the same 6.Be2 and later a4 from white:

Generally. You can prevent a, than you have to allow b. Or prevent a and b then you have to allow c. And b and c may be even worse.

Simple example: play any opening with a4 and your long-castling option has gone forever.

This a4 can turn out to weaken a whole complex like b3, b4, a5.

Of course a4 goes with O-O, not O-O-O.
The main purpose of Najdorf's 5...a6 is not so much to follow up with ...b5, but rather to play 6...e5 without allowing 7 Ndb5.
a4 may weaken b4, but a4-a5 may lame the whole black queen's wing.
This was the favourite of Karpov like #5 and of Geller.

I mean one can try but it is no universal panacea. Some games look like Black had done better refraining from that brain-fart^^

Wise man said :D ... weakening a whole complex like b3, b4, a5