Oral contraceptive pills (OCPs) are hormonal preparations taken mainly to prevent pregnancy, but also to treat conditions such as endometriosis and to regulate menstruation in dysfunctional uterine bleeding.
There are two types. The combined oral contraceptive pill (COCP) contains two hormones, estrogen and progesterone. The progesterone-only pill (POP), sometimes called the "mini-pill", contains only progesterone. Both are discussed here, but most of the concern — and most of the data — applies only to the COCP, the one containing estrogen.
The main concerns
It has long been recognized that women taking the COCP have a slightly higher risk of a blood clot in a leg vein — a deep vein thrombosis, or DVT. About 5 in every 100,000 women using the COCP develop a clot as a result, and every woman starting it should have this explained. If a woman becomes pregnant, though, her risk rises twelvefold, to 60 per 100,000 — so the COCP can be seen as protective, preventing both a greater risk of clots and an unwanted pregnancy. The POP mini-pill carries no added risk of DVT.
There has also been speculation that trekking at altitude is itself a risk factor for clots, particularly if you become dehydrated. The available evidence doesn't support this: there is little to suggest that going to moderate or high altitude for less than three weeks raises your chance of a clot, even if you become dehydrated.
At extreme altitude (above 5,500 m), altitude physicians generally have the impression that the risk of clotting is higher. This is only an impression — no data exist to prove or disprove it. Mountaineers who spend several months at extreme altitude, often in cramped conditions, are the group most commonly thought to be at risk.
Our conclusion
The available evidence suggests women using the COCP run no greater risk of DVT at altitude than at sea level. Women who smoke, however, have a considerably higher risk of DVT regardless of altitude, and COCP use compounds it. There's plenty of anecdotal evidence that women using the COCP while trekking are at no greater risk, and many thousands do so every year. At CIWEC and other facilities that regularly treat travelers to altitude, COCP use doesn't seem to make women vulnerable to clots; the clotting problems we do see occur more in smokers, sedentary people and the elderly. We see no reason to recommend that women stop the COCP because they're going trekking.
Women mountaineers spending prolonged periods at extreme altitude carry a theoretically higher risk, and caution is advised. In these circumstances we recommend considering stopping the COCP or switching to the POP.
Summary
- COCP use increases the chance of a vein clot (DVT), regardless of altitude.
- There is no evidence that altitude alone increases your chance of a DVT.
- The POP (mini-pill) does not increase your chance of a DVT.
- Becoming pregnant raises the chance of a DVT about 12 times more than the COCP does.
- Smoking increases the chance of a DVT considerably, and the COCP or pregnancy compounds it. Smokers planning to use the COCP at altitude should consider stopping it — or, preferably, stopping smoking.
- There is no reason to advise otherwise healthy women to stop the COCP while trekking.
- For prolonged periods at extreme altitude, although there's no evidence of increased risk, we advise caution: consider stopping the COCP, switching to the POP, or using another method if you need contraception.
Using the COCP to avoid a period while trekking
Female trekkers often consider using the COCP to control menstruation. Because periods are so predictable on the COCP, it quickly becomes clear whether one will fall during a trek. You can skip the pill-free week and start the next pack the day after finishing the last pill of the current one. This postpones your period until the end of the second pack (unless you start a third without a break). Doing this once means about six weeks without a period, which will almost certainly let you avoid menstruating on the trek. Taking two packs back to back does not increase the risk of DVT beyond that of taking the COCP itself, and there's no reason not to do it if you wish.
If you have any questions about trekking while taking the COCP, please contact us.