Brief Answer:
Promethazine could reduce further her blood pressure.

Detailed Answer:
Hello!

Thank you for asking on HCM!

I understand your concern, and would explain that promethazine can cause adverse effects like low blood pressure, sedation (in her case it is a needed effect), confusion, disorientation, dry mouth, blurred vision, dizziness, bradycardia, tachycardia etc.

So in her case it would be good for the itching and sleep problems, but it can decrease even more her blood pressure.

From the other hand it has no interactions when taken concomitantly with potassium supplements.

I would recommend trying loratadine instead of promethazine for her itching, which is known to reduce low blood pressure less than promethazine.

But before this, the cause of her itching should be identified. It may be an allergic reaction to potassium supplements, which can lead to further worsening (with severe allergic reaction) if not treated properly.

When did her skin itching start? Was it related to potassium supplements intake? Does she have other symptoms like difficulty breathing, skin rash?

I would recommend consulting with the GP for a careful physical examination and inspection, some allergy tests (cutaneous and blood tests) and some blood lab tests:

- complete blood count ( for chronic anemia or chronic allergy)
- kidney and liver function tests (a kidney and liver dysfunction could cause itching skin and low blood pressure too)
- blood electrolytes (including calcium)
- fasting glucose
- cortisol plasma levels (a adrenal gland dysfunction related to low blood pressure)
- thyroid hormone levels ( a thyroid dysfunction could cause low blood pressure).

Regarding her low blood pressure, a resting ECG and a cardiac ultrasound are necessary to examine her cardiac function and exclude any cardiac related cause of low blood pressure (besides metabolic causes).

What are her mean blood pressure values? Does she feel dizziness when she stands up from sitting position? This would be indicative for orthostatic hypotension and a head up tilt test would be necessary to rule in/out this diagnosis.

Meanwhile I recommend taking plenty of fluids (water and juices), a balanced diet (an increase in salt intake and caffeine would help with increasing blood pressure).

If needed, use elastic support (compression) stockings or tights that cover the calf and thigh. These may help restrict pooling of blood in the legs, thus keeping blood moving, and avoid further decrease of blood pressure.

Hope to have been helpful!

I am at your disposal for further questions, whenever you need!

Best regards,

Dr. Iliri

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