[3] Common side effects by mouth include tiredness, slow heart beat, itchiness, and shortness of breath.

[5] Other side effects include masking the symptoms of low blood sugar in those with diabetes.

[3] Use is not recommended in those with asthma, uncompensated heart failure, or chronic obstructive pulmonary disease (COPD).

[10][11] In its by mouth or oral form, it is used: The combination of timolol and the alpha-1 blocker prazosin has sedative effects.

[3][14] The mechanism of action of timolol is probably the reduction of the formation of aqueous humor[3] in the ciliary body in the eye.

[20] A Cochrane review compared the effect of timolol versus brimonidine in slowing the progression of open angle glaucoma in adults but found insufficient evidence to come to conclusions.

[23] Timolol can also lead to fainting, congestive heart failure, depression, confusion, worsening of Raynaud's syndrome and impotence.