Which drug, when used to treat myasthenia gravis, must be given on time to prevent muscle weakness?

Pyridostigmine is the drug that must be given on time to prevent muscle weakness in patients with myasthenia gravis. It is a reversible acetylcholinesterase inhibitor that increases the availability of acetylcholine at the neuromuscular junction, which is crucial for improving muscle strength. Since myasthenia gravis involves an autoimmune attack on acetylcholine receptors, timely administration of pyridostigmine helps maintain adequate muscle function.

Neostigmine, while also an acetylcholinesterase inhibitor, has a shorter duration of action compared to pyridostigmine, making it less optimal for regular management of myasthenia gravis in terms of timing. Rivastigmine and donepezil are primarily used in the context of Alzheimer's disease and are not first-line treatments for myasthenia gravis. Their mechanism of action, while similar in inhibiting acetylcholinesterase, does not specifically address the neuromuscular problems associated with myasthenia gravis.

Thus, to effectively manage symptoms and prevent muscle weakness in this condition, pyridostigmine's consistent and timely dosing is essential.