Read Aadil Palkhivala's reply:
When a student is on high blood pressure medication, I always suggest that you treat them as if they have high blood pressure. This is because their pressure is not actually reduced internally, but only appears to be reduced externally. In fact, their internal pressure is higher than it was prior to taking the medication. This can be proven (please do not do this) by taking a person who is on high blood pressure medication suddenly off the medication. What happens is that the pressure shoots up, usually way beyond what it was prior to the taking of medication. This means that the body, always seeking homeostasis, is fighting the medication constantly.
Please do no inversions with such students. And yes, Viparita Karani is considered such an inversion, all questions of medication aside. The alternative is Setu Bandha Sarvangasana (Bridge Pose), done on bolsters, which is energizing for the kidneys and hence soothes the system, lowering high blood pressure.
As a side note, I should clarify that there are extreme cases in which high blood pressure medication is necessary. But most of the time it is overused, since high blood pressure can be brought down by changes in diet and lifestyle, and by an individualized series of postures done with pranic breathing.
Recognized as one of the world's top yoga teachers, Aadil Palkhivala began studying yoga at the age of seven with B.K.S. Iyengar and was introduced to Sri Aurobindo's yoga three years later. He received the Advanced Yoga Teacher's Certificate at the age of 22 and is the founder-director of internationally-renowned Yoga Centers in Bellevue, Washington. Aadil is the director of the College of Purna Yoga, a 1,700 hour Washington-state licensed and certified teacher training program. He is also a federally certified naturopath, a certified <a href="/health/ayurveda">Ayurvedic health science practitioner, a clinical hypnotherapist, a certified shiatsu and Swedish bodywork therapist, a lawyer, and an internationally sponsored public speaker on the mind-body-energy connection.