UK Clinical Pharmacy Association


Issues for surgery

For hypertension – loss of blood pressure (BP) control if omitted.

For management of heart failure – potential worsening of heart failure symptoms if omitted.

Risk of hypotension if continued.

Advice in the perioperative period

Elective and emergency surgery 

Continue – monitor heart rate and sodium (see Further information)

Patients taking minoxidil (and sometimes those taking hydralazine) should also be prescribed a beta-blocker (to prevent reflex tachycardia) and, unless receiving dialysis, a diuretic (to prevent sodium and water retention). If it is deemed necessary to stop either of these pre-operatively consideration should be given to withholding minoxidil – discuss with cardiologist. Also see Beta-blocker drug records.

Post-operative advice

Restart post-operatively as soon as next dose is due.

Monitor electrolytes (specifically sodium), renal function and heart rate (see Further information).

Interactions with common anaesthetic agents


Nitrates can increase the risk of hypotension when used concomitantly with inhalational or intravenous anaesthetics.

If hypotension develops use sympathomimetic drugs (e.g. adrenaline/epinephrine, noradrenaline/norepinephrine) with caution as they enhance the cardiac-accelerating effects of hydralazine and minoxidil.

Interactions with other common medicines used in the perioperative period


Concomitant administration of hydralazine or minoxidil with the antiemetics droperidol and prochlorperazine may increase the risk of hypotension.

Further information

NB: following topical application for alopecia systemic absorption of minoxidil is unlikely (only 1-2% is absorbed although greater absorption may occur with use on inflamed skin)

Sodium and fluid retention

Minoxidil causes sodium retention and therefore accompanying water retention. In patients who do not receive dialysis it is essential that patients be prescribed diuretics at sufficient dose to maintain sodium and water balance. Similar effects may also occur with hydralazine in which case concomitant administration of a diuretic may be necessary.

Close attention should be paid to fluid prescribing in the perioperative and post-operative period to ensure sodium and water balance is maintained.


Due to the vasodilator action of hydralazine and minoxidil reflex tachycardia may occur. Concomitant administration of a beta-blocker or other sympathetic nervous system suppressant is recommended to blunt or prevent this response.


Joint Formulary Committee. British National Formulary (online) London: BMJ Group and Pharmaceutical Press. [Accessed on 15th September 2019]

Summary of Product Characteristics – Hydralazine 25 mg Film-coated Tablets. Mylan. Accessed via 15/09/2019 [date of revision of the text September 2015]

Summary of Product Characteristics – Loniten® (minoxidil) Tablets 5mg. Pfizer Limited. Accessed via 15/09/2019 [date of revision of the text June 2016]

Baxter K, Preston CL (eds), Stockley’s Drug Interactions (online) London: Pharmaceutical Press. [Accessed on 15th September 2019]