By Dr Janelle Trees, BSC (HONS), MBBS (HONS), FRACGP
Any powerfully beneficial medicine has potential side effects and will interact with other medication you are taking. Cannabis from medical marijuana clinics is no exception.
Small studies and clinical experience have shown that many patients who take cannabis have a reduced need for the other medications they were taking: either because the other medicine becomes more effective, or the cannabis reduces their symptoms significantly, or both.
Some medicines become less effective when you are taking medicinal marijuana. If you still need that drug (i.e. the medical cannabis doesn’t treat what that medicine treats) you may even need a slightly larger dose. Hopefully, the medical cannabis will improve your quality of life (including allowing you to reduce or cease other drugs) so that this is acceptable.
On the other hand, some medications may be metabolised more slowly in the presence of cannabis. This can make their effect stronger and increase the risk of side effects. Examples of such medicines include: Proton pump inhibitors (very commonly prescribed meds which reduce stomach acid); Warfarin—taken for heart conditions or by people who have had a blood clot, for example; and some anti-seizure medications—taken by people with epilepsy or other seizure disorders or sometimes for nerve pain.
Cannabis as medicine can also interact with some allergy or cold medications and other over the counter medicines like St John’s Wort, a herb used to treat depression.
Your GP has a key role, working with another specialist and/or the team at an Australian medical marijuana clinic, to help you reduce, substitute or cease other medicines, as you find what type and dose of medical cannabis suits you. Be aware that it usually takes a week or two to find your optimal dose of medical cannabis.
It is quite likely that you won’t need so much of some of your regular medications if the cannabis is working for you. But you must never stop medications cold turkey. Many drugs need to be reduced slowly over time—to stop them suddenly may lead to unpleasant or even dangerous side effects.
Some antidepressant medications, sleeping pills and tranquilizers interact with medical marijuana. Again, don’t stop taking these medicines suddenly, but work with your doctor to reduce your dosage as you titrate your dose of cannabis up to the level where you achieve maximum benefit with minimal side effects. Side effects of medicinal cannabis, incidentally, were found to be mostly minorin a 2015 study.
The two active ingredients in cannabis that we work with are THC and CBD. They are called cannabinoids. The marijuana plant has over 70 different cannabinoids, but THC and CBD are the ones we know most about. CBD, used for arthritis pain, for example, inhibits an enzyme in the liver which breaks down some medicines and eliminates them from the body
Because of this, some medicines that could make your medical marijuana more concentrated include: macrolides (a class of antibiotics), antimycotics (anti-fungal medications), calcium antagonists, HIV protease inhibitors, amiodarone (which is taken by some people who have a cardiac condition) and isoniazid (an antibiotic usually taken for TB treatment). This means you should start at a smaller dose and increase your dose of medical marijuana even more slowly than most.
Other drugs which reduce the effect of THC include: rifampicin and rifabutin (more antibiotics), carbamazepine, phenobarbital, phenytoin, primidone (all of which are anti-seizure meds), and troglitazone (a diabetes drug). If you need to take one of these, your GP may need to guide you to a relatively higher dose of your cannabis medicine.
Cannabis increases the effective amount of other drugs that is available to the body. Such medicines include: amitriptyline (used for depressive anxiety disorders, ADHD and other mental illnesses), theophylline (a drug for the lungs), granisetron (an anti-vomiting drug used for chemotherapy patients), dacarbazine (also called imidazole carboxamide—a chemotherapy drug), and flutamide, used to treat prostate cancer, among other things. These drugs may need to be closely monitored and sometimes slightly reduced.
You can see from this (probably incomplete) list that drug interactions with your medical cannabis can be a complex business. Your GP will help you.
Because it is still early days of the use of medical marijuana in Australia —most of us weren’t taught about this at medical school—your GP may choose to work closely with another specialist or the medical marijuana clinic near you. Your medical marijuana prescription needs to be made in the context of your medical history and your current medications. This way, you can be safely on your way to relief of pain or other symptoms and a more comfortable and fruitful life.