All mountain walking presents hazards, but in the high regions, the biggest danger comes from the dehydrating high altitude, severe cold, and bitter winds. Be careful on the rough and rocky trails; bear in mind that the next hospital is miles away. The real problems of altitude sickness occur at heights above 3,000m especially if you climb quickly.
There are generally two levels where the effects of altitude kicks in, 3,500m and again around 4,500m so these two stages of upward motion should be carefully planned. Even before you reach 3,000m, it is beneficial to gain height as slowly as possible, including extra nights. The recommended daily height gain is only 300m, with 500m, per day being the upper limit. Obviously, in some areas, this is difficult, so you must monitor your condition closely.
Serious altitude problems occur in the highest mountain regions of northern Nepal. Kathmandu is at 1,350m and presents no problems. To combat the problems of altitude, it is important to learn about its effects before you start hiking along the trails. The most common symptoms of altitude are headaches, nausea, tiredness, lack of appetite and disorientation. it is often difficult to sleep and breathing becomes erratic (Cheyne-Stokes breathing). The heart might thump a bit disconcertingly at times. Be very careful not to over extra on arrival at any destinations; symptoms often only being to appear after an hour or more. Having some of these doesn't mean abandoning the trek, but watch for any changes as altitude is gained. mild symptoms, perhaps just a slight headache, are acceptable so long as they do not get worse or persist all day and night. One real nuisance is the need to urinate more, especially at high camp, but it's a good sign as a rule!
It is vital to walk very slowly at altitude, especially when climbing any hill. If in doubt, be sure to admit any problems and don't be pressurized by your fellow trekkers. Altitude sickness does kill. If you are having any serious effects before a high pass, it might require another intermediate night or at worst an immediate return downhill. Minor effects of altitude above 4,500m are felt by most-nausea and extreme lethargy but symptoms will improve on the descent. Continuing to ascend with any persistent symptoms can lead to the serious risk of pulmonary and cerebral edema or even death. Deaths occur each year in the Himalaya and Nepal, despite all the warnings. Complications from altitude sickness can strike very quickly. As an aid before the trek, some start a course of Diamox a diuretic which thins the blood, making you urinate more which is generally considered good at altitude. it can have the disturbing side effect of pins and needles in the fingers. Another option is to try homeopathy coca is available as homeopathic tablets that some trekkers ( including the authors) swear by. Others swear by ginkgo biloba tablets, which appear to walk for an unfathomable reason. It is suggested that these can be taken twice a day for five days before arrival and one tablets a day during the trek. However, there are some side effects when taken which some prescription drugs and other substances so check with your GP.
A Gamow bag is a larger bag used to temporarily relieve the effects of high altitude. An ill person with serious altitude problems can be cocooned under higher air pressure for a limited period to mimic a lower altitude. Oxygen cylinders are generally available in the Everest region, partially in the Annapurnas, but elsewhere only if carried in by trekking crews. Generally, it's better to proceed slowly and try to avoid any problems.