Botulinum toxin is an exotoxin produced by the bacterium Clostridium botulinum. It is widespread in the environment and in anaerobic conditions of the intestines of many animals, where it is mostly found in the form of spores. It is considered one of the most toxic biological substances known to man. We distinguish seven serotypes of botulinum toxin. All serotypes show a similar molecular structure although there are variations in the amino acid sequence. Botulinum toxin works by interfering with nerve signal transmission by blocking the release of acetylcholine from presynaptic motor neurons. Acetylcholine is the main neurotransmitter of the neuromuscular junction, and the inability to release it causes muscle paralysis. Due to the difference in site of action of different types of toxins, the duration of the blockade is also different, and consequently clinical applications of the toxin. The muscle release effect seen with botulinum toxin serotype A lasts 4-6 months, which is longer than with other serotypes. The use of botulinum toxin serotype A in ophthalmology can be anatomically classified into three categories: glabellar area, extraocular muscles, lacrimal gland. There are a number of potential complications associated with clinical use of botulinum toxin A. Most of these complications are very rare and result from the spread of botulinum toxin through the bloodstream with adverse effects on surrounding and distant muscles that were not targeted. However, just as the action of botulinum toxin itself is not permanent, all complications and side effects are also reversible within a few weeks after application. Contraindications to use of botulinum toxin are divided into absolute: known hypersensitivity to any component of the drug, and relative, which include hypersensitivity reactions, generalized disturbances in muscle activity, concomitant use of aminoglycoside antibiotics, infection at or around the application site, bleeding disorders, anticoagulant therapy, pregnancy, lactation and botulinum toxin use in children under 12 years of age.