Promethazine blocks the effects of a natural chemical called acetylcholine. This can help dry up a cough or runny nose, but can also cause side effects such as a dry mouth and dry nose. Some cough and cold remedies claim to suppress your cough or stop you bringing up phlegm. Although some people find them helpful, they're not usually recommended. This is because there's little evidence that they're any more effective than simple home remedies and they're not suitable for everyone.