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Comment

I enjoyed this short report as it reminds me of just how humble we need to be with the subject of minoxidil-related side effects. There is a short list of minoxidil side effects that everyone learns about – and then there is the long list of rare side effects that not everyone ends up learning about and few ever talk about.

The common side effects that everyone needs to know about (ie the “short list”) include hair shedding in the first few months, hair growth on the face/body, dizziness, headaches and heart palpitations. Uncommon issues of topical minoxidil include chest pain, fluid retention.

Paresthesias are a rare side effect and I have seen this many times. In fact, I’ve seen patients who have been evaluated for a range of neurological conditions after using minoxidil without realizing that it was probably the minoxidil. This includes patients who have had work ups for issues like multiple sclerosis after use of topical minoxidil. The symptoms and clinical presentation overlap with carpel tunnel syndrome given the distribution in the area affected by the median nerve.

I don’t think we fully understand why some patients get numbness and paresthesias with minoxidil or really how rare this even is. Is it a direct effect of minoxidil? Is it due to fluid retention? Is it more likely in those who use their hands and have some predisposing issue in the carpel tunnel? Does levels of vitamin B12 affect this issue?

We certainly need to be aware of minoxidil paresthesias as a rare side effect. This is particularly important as more and more clinicians around the world are prescribing oral minoxidil.

This case does not prove that minoxidil caused this reaction. But it does hint at a probable association. If you are not familiar with the Naranjo scoring system – you probably should be! Especially if you are a clinician and are involved with thinking about these sorts of side effects and how likely they are. For this case, the Naranjo score I calculate is 4 which means “probable” side effect.

A Closer Look at the Naranjo Adverse Drug Probability Scale

The Naranjo Scale was created nearly 40 years ago to help standardize how clinicians to about assessing whether or not a drug could be implicated in an adverse drug reaction. It is used in controlled clinical trials. The scale is quite easy to use – and involves asking the patient 10 questions. Answers to the question are recorded as “yes”, “no” or “don’t know” and different points are assigned to each answer (-1, 0, +1, +2).

Typical Questions in the Naranjo Scale

1. Are there previous conclusive reports on this reaction?

Yes (+1) No (0) Do not know or not done (0)

2. Did the adverse events appear after the suspected drug was given?

Yes (+2) No (-1) Do not know or not done (0)

3. Did the adverse reaction improve when the drug was discontinued or a specific antagonist was given?

Yes (+1) No (0) Do not know or not done (0)

4. Did the adverse reaction appear when the drug was readministered?

Yes (+2) No (-1) Do not know or not done (0)

5. Are there alternative causes that could have caused the reaction?

Yes (-1) No (+2) Do not know or not done (0)

6. Did the reaction reappear when a placebo was given?

Yes (-1) No (+1) Do not know or not done (0)

7. What the drug detected in any body fluid in toxic concentrations?

Yes (+1) No (0) Do not know or not done (0)

8. Was the reaction more severe when the dose was increased, or less severe when the dose was decreased?

Yes (+1) No (0) Do not know or not done (0)

9. Did the patient have a similar reaction to the same or similar drugs in any previous exposure?

Yes (+1) No (0) Do not know or not done (0)

10. Was the adverse event confirmed by any objective evidence?

Yes (+1) No (0) Do not know or not done (0)

Determining the Naranjo Score

Scores can range from -4 to + 13. A score of 0 or less means the likelihood of the drug causing the side effect is doubtful, a score 1 to 4 indicates it is ‘possible’, a score 5 to 8 means it is ‘ probable’ and a score 9 to 13 means it is ‘definite’

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