Botox injection, when used to treat migraines, can cause side effect including headache and eye lid drop, but not myasthenia gravis. In myasthenia gravis, a person's immune system produces antibodies, which work as the blocker between the nerves and muscles. A person with migraine and without myasthenia gravis has normal communications between the nerves and muscles, and that communication is the cause of migraine, so Botox is injected acting as the blocker. The results are similar, muscles can't accept the signals from the nerves. But the blockers are different in the two scenarios.