The Financial Ombudsman Service (FOS) said alcohol exclusions are a common feature of travel insurance policies and if an insurer thinks someone's accident was due to excessive alcohol consumption it may not pay out.
The service gave the warning as it released its latest complaints figures, for the final three months of 2017.
It said it looked at nearly 900 travel insurance complaints in the last three months of 2017.
The FOS said a few claims it examined had been made after the insurer had decided someone's drinking was a factor in an accident they had while on holiday. Some also related to specific types of holiday activities, such as winter sports or cruise breaks, which may not be included in a standard policy.
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The FOS is reminding people to check the small print of their policies, saying insurers can be very specific about what they will cover.
If a consumer makes a claim on their travel insurance, the burden of proof falls on the insurer - not the consumer - to show how any exclusion in a policy applies, the ombudsman service said.
In one recent case, the ombudsman told an insurer to pay up for a man's medical treatment costs after he had slipped over in a nightclub's toilets while on holiday, hitting his head.
The man said he had been drinking - but that he was not drunk at the time.
The ombudsman decided that, on balance, it did not think the evidence showed it was more likely than not that excessive alcohol consumption had caused his accident.
The service said figures show more than 21.9 million people from the UK went on summer holidays abroad last year.
By comparison, the ombudsman service dealt with with around 3,000 complaints about travel insurance in 2017 - deciding in nearly four in 10 cases that the insurer in question had not treated their customer fairly.
It said that, encouragingly, compared with recent years it is generally upholding fewer travel insurance complaints - suggesting many insurers are increasingly treating their customers in a fair and reasonable way.
Caroline Wayman, chief ombudsman and chief executive at the FOS, said: "Insurers may choose not to pay out if they believe someone's been drinking excessively - although this doesn't mean holidays need to be totally alcohol-free.
"In each case we look at, we'll carefully weigh up all the evidence to decide, on balance, whether the insurer has made the right call."
A spokeswoman at the Association of British Insurers (ABI) said: "Travel insurance is a lifeline for people who run into trouble overseas, with insurers paying out more than a million pounds every day.
"More than half of this funds emergency medical treatment for people who have been badly injured or have fallen seriously ill.
"As with any insurance, customers do have a responsibility not to behave recklessly.
"Insurers know people will likely want to drink alcohol while they're on holiday and they don't expect you to stay sober all the time, but there is a danger of invalidating your cover if you drink so much that it makes you act dangerously or means you are out of control."
Here are the FOS's top tips for buying travel insurance:
1. Buy the right cover for you - do not just look at the price of travel insurance.
2. Booking your travel insurance at the same time as you book your holiday will mean you have a policy in place in case your circumstances change and you need to cancel your trip.
3. Do not pay the price for drinking too much. Alcohol exclusions are a common feature of travel insurance. You are not expected to avoid alcohol completely - but bear in mind your claim may be turned down if it is linked to drinking excessively.