Avomine is the brand name; the name of the medicine it contains is promethazine. Being a pretty old drug, it has been used over the last few decades and is known to be relatively safe. Due to extensive use, its side effects are also known. However, its use during pregnancy is not recommended due to lack of evidence of safety in pregnant humans. Its use in animals during pregnancy has produced no untoward effects. The drug reaches the fetus, which is not desirable. It also appears in mother's milk and hence should not be used during lactation.
Its side effects are not serious, except that since it is an anti-vomiting agent, vomiting due to some serious causes (such as food poisoning, brain fever, etc.), not related to motion sickness, may go unnoticed and hence untreated. Therefore, if there is nausea and / or vomiting not related to travel, it must be investigated. For motion sickness, the drug should be taken one hour before the time of travel.