'I read that taking an aspirin daily reduces the risk of heart attack. Why not take two a day and double the protection?' Which fallacy is committed in the above statement?

The reasoning commits the fallacy of quantity, the mass fallacy.

It assumes that if one aspirin gives a benefit, two must give twice the benefit.

The favourable effect of a small dose is treated as if it scaled up in simple proportion.

Doubling the quantity may in fact bring harm rather than doubled protection.

Wrongly transferring a property of a certain amount to a larger amount is the quantity fallacy.

The error lies in assuming benefit multiplies straightforwardly with dose.