Taking amphetamine-like substances such as beta-ketones is associated with blurred vision. This is different to distorted vision caused by pupil dilation. This kind of blurred vision, where there is no pupil dilation, is associated to relaxation of the ciliary muscle of the eye.
The ciliary muscle is a ring of striated smooth muscle in the eye’s vascular layer that has several roles including control of the fine focus of the lens in the eye.
When the eye needs to fine focus at times of reading or looking at your computer screen, the contraction of the ciliary muscle causes compression of the eye lens to accommodate this. Relaxation of the ciliary muscle returns the lens to the relaxed state.
The process of accommodation mediated by the ciliary muscle changes the optical power of the lens to maintain focus (clear sharp image) on objects at variable distances.
Typically closer or smaller objects require more effort to see which is achieved by contraction of the ciliary muscle compressing the lens.
Ciliary muscle has sympathetic and parasympathetic innervation. This means that the sympathetic (fight and flight) and the parasympathetic (rest and digest) pathways are involved in the regulation of relaxation and contraction of the muscle.
The dominant innervation is the sympathetic. This means that if both pathways are simultaneously active only the dominant pathway, in this case sympathetic, has the final control.
Activation of the beta-2 (sympathetic) nerves of the muscle cause the ciliary muscle to relax and the activation of muscarinic M3 (parasympathetic) nerves cause the muscle to contract and compress the lens to the extent needed for focusing.
Taking too much amphetamine like substances lead to excessive activation of the sympathetic pathway and the excess noradrenaline (norepinephrine) released leads to the constant activation of the beta-2 sympathetic receptors of the ciliary muscle and as the sympathetic pathway is the dominant one in that muscle, the muscarinic pathway is left unable to focus the eye by contracting the ciliary muscle.
Some people may have noticed that looking at arousing pictures / music videos may lead to the eye to temporarily gain the ability to focus on the image. This is because the serotonin pathway has a neuro-modulatory effect on the ciliary muscle. In our very early (non-primate) evolutionary history muscles used to be controlled by the serotonin pathway and this can re-adjust the balance of nervous activity at the ciliary muscle.
Miotics such as Pilocarpine (eyedrops only) being a non selective M3 muscarinic agonist works on both the ciliary muscle and the iris sphincter muscle and maybe helpful though carry side effects such as iris muscle spasm which can lead to headaches.