Detained patients have to apply for Section 17 leave in order to exit the hospital to vote, or they can vote by proxy or by post.
Only patients convicted of committing a criminal offence cannot cast a ballot.
Yet the turnout for those with mental health problems is often very low.
The most accurate survey, from the 2010 General Election, showed inpatients in the central London area of Westminster were 50% less likely to register than the general public, and those who did were a further 50% less likely to vote.
According to the journal The Psychiatrist, a "significant number" of mental health service users are either unaware of their eligibility, do not know how to register or lack the confidence to vote.
Louise understands why. She was nervous walking up to the polling station as she thought she might be turned away.
"There's that conception you don't understand what you're doing, that you shouldn't actually be there," she said.