Drug Interactions

I also faxed 2 different doctors’ offices today regarding drug interactions potentially leading to serotonin syndrome. They made a big deal about serotonin syndrome in pharmacy school, but I didn’t actually see it until I was on my medicine rotation at the VA. The patient had to be adminstered cyproheptadine and cooled with ice packs. It was scary and totally preventable. So now I always watch for potential causes of it.

The first patient is currently taking dextroamphetamine and tramadol and was just prescribed paroxetine for anxiety. He had been taking buckets of alprazolam, so the doctor choosing an SSRI was a good idea to prevent anxiety. But with three serotonergic agents, the possibility of serotonin syndrome was there. I called the doctor’s office and they told me to fill the prescription anyway. I did, but talked to the patient about the possibility of serotonin syndrome and what to watch for. I also advised him not to use OTC cough and cold products that contain pseudoephedrine and to call me or his doctor if he had a question about what he could or could not take. I sent a fax to the doctor’s office advising them (in writing) of the interaction and about my conversation with the patient.

The second patient was a guy stabilized on sertraline. His doctor prescribed phentermine for weight loss, even though this guy is NOT fat (in my book at least). When I went over to counsel him I asked him, jokingly, what he was using the phentermine for. We talked about weight loss and I gave him some pointers on how to not to mess up his metabolism taking amphetamines (not to take it every day, etc) by trying not to become accustomed to the medication. I also gave him my serotonin syndrome talk and told him to say no to pseudoephedrine, triptans for migraine, other antidepressants (especially MAOIs) and tramadol for pain. I sent a fax to his doctor, too.

I usually only get a NEW drug-drug interation potentially leading to serotonin syndrome once a month or so. To get 2 in one day is a little strange. Most of the drug interactions that could cause serotonin syndrome are medictions that patients have been taking concurrently for months or years and they only ever have problems if they add a third or fourth serotonergic agent.

Drug interactions are difficult to navigate. Obviously some interactions are more serious than others, and some potentially lethal interactions are relatively rare. In some cases it is important to speak to the prescriber (ok, ok, the prescriber’s receptionist) before dispensing the offending drug, and sometimes I think informing the patient and faxing the prescriber will do. I know that when I do contact a prescriber’s office regarding a drug interaction, it usually takes them at least a week to return my call, and 99% of the time they say to dispense the medication anyway.
Or, as I’ve learned through experience, they change the drug to something else in the same family.

I fax on interactions between beta-blockers and calcium-channel blockers all the time.

Recent example: small elderly woman has been taking metoprolol and her doctor tried to add verapamil. Calcium-channel blockers can increase the pharmacologic effects of beta-blockers leading to symptomatic bradycardia (low heart rate), and they should be added with caution. The doctor had prescribed 240mg of extended-release verapamil without an initial trial/titration with a smaller dose. When I called on this interaction (and to suggest a smaller inital dose), the doctor just changed the medication to diltiazem XR 240mg.

Another non-dihydropyridine calcium-channel blocker.

With basically the same drug interactions. Granted, the diltiazem is a little bit better than the verapamil.

I dispensed the diltiazem to the patient after talking to her at length, and sent a fax to the doctor’s office about the interaction and my conversation with the patient. I told her to call 911 if she felt like her heart was beating slower than usual accompanied by dizziness and to call her doctor if she had any other side effects.

I hope nothing happens to her.