If you didn't know, the Himalayas are a little high and being this far from the centre of the planet can lead to altitude sickness. Annoyingly, this can affect anyone at almost any time, and whilst most of you will be totally fine, there is the chance that someone gets clobbered with it, and it should be taken seriously. We joke a lot because we're generally a bit silly, but altitude sickness is about as much fun as your head exploding because that (sort of) what happens.
In brief, high altitude can be deadly, and it's difficult to predict who will be affected. Curiously, being young, male, physically fit, and drinking alcohol all slightly increase your chances of suffering from it.
Taking time to acclimatise is crucial, especially if you're starting your adventure from Leh - which is already at 3500m above sea level., and you'll be heading over the highest pass on your first day of driving. Drugs like Diamox can help speed up acclimatisation - they can be acquired in Leh easily, though Shimla doesn't appear to as easy to source them in. As a preventative, the recommendation is to start taking it 1 day before going to altitude and for 2 days after reaching altitude.
** Please be aware - this is a guide only - if you have any concerns, doubts or questions then speak to your Doctor **
Are you more of a details kind of person? Then read on and shit your pants accordingly. Before you go running off to mummy though, bear in mind that thousands of people travel this route annually with the majority suffering no, or only very mild symptoms.
Ready? Okay, let's science the shit out of this...
At 5500m atmospheric pressure is about 50% of sea level pressure. The result is less oxygen getting into your blood, which will deprive the body of adequate oxygen supply (Hypoxia).
With time your body has a variety of ways it can acclimate to lower air pressure to avoid hypoxia - ranging from increased respiratory rate (2-3 days) to increased oxygen-carrying haemoglobin in the blood (weeks) to increased capillary density (weeks to months).
If your body experiences a sustained period of hypoxia the veins in your brain begin to dilate causing fluid to leak into your brain (Cerebral Edema). The increased intracranial pressure will initially cause headaches, nausea and blurred eyesight. Eventually, it will cause the brain to herniate most likely killing you.
Conversely, the veins in your lungs constrict, causing them to leak fluid which accumulates in the lungs (pulmonary oedema). Initially, this could exhibit as a cough or wheezing. Eventually, it will cause respiratory failure and very quickly after that death.
Drugs can speed up part of the acclimatisation process and but are limited in how much they can do. Respect the altitude and listen to your body. Understand the symptoms and then be prepared to seek lower altitude if things are going tits up.
Still not enough for you? Then here's a video that is so full of science it makes Einstein's back catalogue look like a pamphlet: