I suspect so. Evidence from multiple scientific and medical sources indicates a clear association between chronic alcoholism and a decline in IQ. Obviously correlation is not causation, and it's hard to argue we should do an RCT for drinking with any medical ethics board. But in the case of a small molecule neurotoxin like alcohol that so easily weaves its way into and through brain tissue, and with relatively easy-to-understand molecular interactions, it seems likely to be causal.
Alcohol and its metabolites directly harm and kill nerve cells, contributing to a reduction in brain volume. Here it's particularly important to remember that neurons, if they grow back at all, don't do so anywhere near the rate of, say, your skin cells. But it's also true that alcohol use can damage blood vessels, including in the brain. It's not hard to suspect that causes less efficient neural activity over time even if no actual neurons are killed.
