Myofascial pain syndrome is a painful muscle condition which is defined as local or referred pain associated with hypersensitive palpable nodules in the way of that muscle and is a frequent pathology in consultations on chronic pain. Botulinum toxin is an exotoxin produced by Clostridium botulinum, of which serogroups A, B and F have clinical utility (mainly type A-Botox ® , Dysport ®). This is one of the most potent neurotoxins and is administered compassionatelly in the treatment of myofascial pain syndrome. The toxin has a beneficial analgesic effect by reducing muscle hyperactivity but recent studies suggest that this neurotoxin may also induce analgesia by non-neuromuscular actions. Its use is not without risks. Adverse effects are related to the migration of the toxin and are usually mild or moderate and transient. There have been reports of prolonged muscle weakness and diseases like myasthenia gravis or Lambert Eaton syndrome triggered by the use of the toxin. We report a case of a male patient with myofascial pain syndrome whose psoas and quadratus muscles were treated with injections of botulinum toxin. Following the improvement in clinical muscle situation, repetitive diplopia and ptosis developed and the patient was diagnosed of myasthenia gravis. We review the pathogenesis of myofascial pain syndrome and myasthenia gravis and the use of botulinum toxin and relationships between them. We conclude that a detailed history is required prior to the use of botulinum toxin on patients with muscle pathology or signs of muscle weakness.