In theory, more than 30 drugs currently used for many different conditions can interfere with nerve → muscle transmission. Nature has made the ignition system very secure. Normally there is such a safety factor that, even if a drug interferes by up to 50 per cent, there is still enough in reserve to prevent any weakness. The problem in myasthenia is that the reserves are low. Only a very slight further dip in transmission can make things a lot worse.

Below is a list of drugs that can make myasthenia worse; while reading it, try to remember these very important general guidelines:-

  1. When the myasthenia is well controlled, the safety factor of nerve→muscle transmission has been largely restored, which is why the patient is stronger. If so, the listed drugs are unlikely to cause a major problem. Mild worsening may be noticed and one must keep on the lookout.
  2. Obviously, therefore, patients with poorly controlled myasthenia are most at risk from such drugs. Infections (e.g. pneumonia) can make myasthenia worse (myasthenic crisis). If it is a serious infection, then one of the powerful antibiotics on the list may have to be used, with the risk of making the myasthenia worse still. The hospital specialists need to be fully aware of all of the potential problems and must be ready to deal with them.
  3. Any reaction to any drug can be very specific to the individual patient – including worsening of myasthenia. Some drug reactions are rare and most people with myasthenia would never get them.
  4. Some drugs have been proved to affect nerve → muscle transmission. Many others are suspects but have never actually been tested. Indeed our only evidence against them may be that one MG patient became weaker after taking it. Myasthenia weakness can vary a lot from day to day without any obvious cause, so some drugs have probably been blamed unfairly for a dip that was purely coincidental.
  5. The take-home message is very simply to check the list of suspect drugs when starting any new one. If it is there, both the MG patient and the prescribing doctor should be aware that it might worsen the patient’s myasthenia. They should also remember that, with any drug, they might be the first to notice a problem, especially if that drug is an unusual one, or a newcomer. Its absence from the list is not a cast-iron guarantee of an easy ride.
  6. The tables give warnings, not a list of banned drugs. While doctors will always try and use safe alternatives, some of those on the list may be life-saving (e.g. for a resistant germ). If they really are needed, it may still be possible to control any effects on the myasthenia
  7. All drugs have what is known as a proper, or generic, name. These are sometimes rather long and complicated and drug manufacturers use shorter names, not least so that they can patent them. A single drug may be known by literally dozens of brand names. Also, drugs are sometimes combined and prescribed under a name that doesn't clearly indicate the individual components. It is now recommended that doctors only use proper (generic) names of drugs - the only ones used here. If popular trade names were used there would be a danger of omissions, also, such names may come and go.

Every drug, whether prescribed by the doctor or bought over the counter, has to be labelled with the proper name - so check that

Antibiotics & Antimalarials Beta-Blockers Other Heart Drugs Drugs used in Neurology & Psychiatry
Acrosoxacin Acebutolol Procainamide Chlorpromazine
Amikacin Antenolol Quinidine Clozapine
Azithromycin Betaxolol Flupenthixol
Cinoxacin Bisoprolol Isocarboxacid
Ciprofloxacin Carvedilol Lithium
Chloroquine Celiprolol Loxapine
Doxycycline Esmolol Methotrimeprazine
Erythromycin Labetolol Oxypertine
Gentamicin Metoprolol Pericyazine
Hydroxychloroquine Nadolol Perphenazine
Kanamicin Oxprenolol Phenelzine
Lymecycline Pindolol Pimozide
Minocycline Propranolol Prochloperazine
Naladixic Acid Sotalol Promzine
Netilmicin Timolol Phenytoin
Norfloxacin Risperidone
Ofloxacin Sulpiride
Oxyytetracycline Thioridazine
Streptomycin Tranylcypromide
Telithromycin Trifluoroperazine
Tetracycline Zuclopenthixol

Antibiotics and anti-malarials

These are used to treat infections are amongst the commonest drugs prescribed by GPs. Remember that the infection rather than the drug might be worsening the myasthenia and it may be hard to know which to blame. The antibiotics most likely to worsen myasthenia are usually given in hospital, by injection, for very serious infections. The one exceptional drug that must be avoided in myasthenics is the ketolide antibiotic, Telithromycin (Ketek) which has caused deaths.

Drugs for malaria are also sometimes used to treat rheumatic problems.

Drugs for the Heart

Beta blockers are widely used to treat high blood pressure and angina and occasionally also for anxiety. A fairly common aide -effect is a feeling of tiredness but in rare cases they can specifically worsen myasthenia. New ones are being introduced all of the time so this list may not be complete. Most of their names end in -olol. 

Drugs used in neurology and psychiatry

These drugs are used for a wide range of neurological and psychiatric disorders. For most of them, the evidence that they cause problems in myasthenia is limited and none are absolutely banned. It is sensible to be cautious, as with all new prescriptions.

Muscle Relaxants


Typical myasthenia can appear in some rheumatoid arthritis patients who are treated with the drug penicillamine. This is therefore best avoided.