The advice follows news that the NHS in England has bought more than half a billion disposable cups over the last five years.
Figures obtained by the Press Association show 174 acute, mental health and community trusts have purchased more than 600 million since 2013.
That's the equivalent of more than 330-thousand cups per day.
Andy Lester from the Christian charity told Premier people could very easily help reduce the figures.
"If you're going for a routine hospital visit and you know you're going in there, as inconvenient as it may seem, take your own cup," he said.
"There are plenty of really attractive cups you can take - sling over your shoulder, put in your bag and take with you and that helps to save the planet. Those are small steps but they're important steps and if we were all doing that every time we went to the hospital...we're saving on [using] a cup."
One London trust, Guy's and St Thomas', purchased almost 30 million cups over a five-year period, with 6,258,249 purchased in one year alone.
The stats come amid rising concern over the environmental impact of single-use throwaway items.
Cups are used across the health service from waiting rooms to wards.
Plastic, foam and paper cups have become entrenched in NHS procurement.
Greenpeace UK said the figures demonstrate "just how out of control our relationship with single-use plastic has become".
Echoing this point, Lester told Premier his message for the health service: "This is an opportunity for you to lead by moving to biodegradable alternatives.
"There are fully compostable alternatives out there - cups that can be put into landfill and will eventually degrade. Let's do everything we can to move away from single-use plastic."
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