The world of disease modifying drugs is a complete balancing act, you have to balance the benefits against the potential side effects.
For a first line drug, Tecfidera is pretty good really, it’s got that 50% average relapse reduction rate and the side effects are mostly handled quite well. The most common is nausea and upset stomach particularly at first, but you can try to get a slower introduction to the drug to help that. I used to have a link to a page on the Barts Blog (very well reputed and reliable), which explained that Tecfidera works best when it’s the first drug a person with RRMS has tried. But the link seems to have vanished!
But as to the side effects you mention: yes it can lower your lymphocyte count. It’s actually expected to do this initially, but then they should return to ‘normal’ levels. It’s when they stay low or keep going down that the problems begin, and then you are taken off the drug. Your blood is tested very regularly in the first year particularly, so you aren’t left with low lymphocytes and at risk.
The low lymphocytes (aka lymphopenia or lymphocytopenia) is problematic because lymphocytes are the type of white blood cell that fights against viruses. The particular virus the medical professionals are concerned about is, as you’ve mentioned, PML. This is a very nasty virus that attacks one’s brain. You can only contract PML if you have antibodies to another virus called John Cunningham Virus (JCV). JCV is very common and in general people are unaware they’ve come into contact with it. Tysabri is another DMD which can cause PML, but JCV status is checked before you start that.
I imagine the risk is very much less with Tecfidera, and managed better with blood tests for lymphopenia. The fact that your blood is checked so regularly is a fairly decent failsafe. It takes a while - several months - for your lymphocytes to recover, but they do.
There is a decision aid tool on the MS Trust website: https://www.mstrust.org.uk/about-ms/ms-treatments/ms-decisions-aid That might help with your decision making.
Personally I’ve had a very poor relationship with DMDs. Avonex gave me cognitive problems, Copaxone stopped working after a few years on it and I started relapsing again (but I had no side effects). Tysabri gave me hepatitis (very low liver enzymes, but from which I recovered - the liver is an amazing organ). Then I tried Tecfidera, and had very low lymphocytes after about 6 months and had to stop the drug.
I’m now in a stage that was called secondary progressive for about 2 years, but is now progressive relapsing, after two relapses last year. I’m now ineligible for DMDs because of my diagnosis and also my level of disability. Plus, my history of side effects meant most drugs were ruled out.
In spite of my experience with the drugs, I definitely believe that someone diagnosed with RRMS is better on a DMD than not. Because of my constant failures with DMDs, I wasn’t on one and subsequently had a very bad relapse in early 2012. I haven’t been able to walk properly since then. The damage that constant relapses has done to me has affected every part of my body, from my brain to my feet, including bowel and bladder as well as cognitive problems and of course, being a full time wheelchair user.
If you want a first line drug with almost no side effect potential, you could always opt for Copaxone it’s an injectable, but they’re tiny little subcutaneous needles. The expected relapse reduction rate is only 30 - 35%, but if you’re reluctant to take Tecfidera, it’s an alternative.
If a person can avoid relapses and their severity, they avoid some of the damage to the nervous system. I’d definitely recommend them. You just need to find the right drug for you.