Venenoso vs. peçonhento
In Portuguese, the words venenoso and peçonhento both relate to toxins and danger, but they describe entirely different mechanisms of how those toxins are delivered. The distinction mirrors the biological difference in English between poisonous and venomous.
Venenoso
B1The word venenoso refers to poisonous plants, animals, or substances that contain toxins but lack a specific mechanism to inject them into another organism. You generally suffer harm from a venenoso organism if you swallow, inhale, or touch it, and the word is also widely used in a figurative sense to describe toxic people, comments, or relationships.
Aquele sapo colorido não deve ser tocado porque é muito venenoso.
(That colorful frog must not be touched because it is very poisonous.)
Muitas pessoas não sabem identificar qual cogumelo da floresta é venenoso.
(Many people do not know how to identify which forest mushroom is poisonous.)
A rainha má ofereceu uma maçã com um líquido venenoso para a princesa.
(The evil queen offered an apple with a poisonous liquid to the princess.)
Fique longe daquele homem porque ele tem um comportamento venenoso.
(Stay away from that man because he has a poisonous behavior.)
Algumas plantas de jardim são perigosas se o seu cachorro comer alguma folha de um arbusto venenoso.
(Some garden plants are dangerous if your dog eats a leaf from a poisonous bush.)
Peçonhento
C1The term peçonhento means venomous and is strictly a biological word used for creatures that possess specialized anatomical structures, such as fangs or stingers, to actively inject venom into their prey or attackers. This word is typically used to classify dangerous animals like snakes, scorpions, spiders, and bees.
O fazendeiro encontrou uma cobra silvestre e chamou os bombeiros por medo de ser um animal peçonhento.
(The farmer found a wild snake and called the firefighters out of fear that it was a venomous animal.)
Os escorpiões são os animais de categoria de bicho peçonhento que mais causam acidentes no meio urbano.
(Scorpions are the animals in the venomous creature category that cause the most accidents in urban environments.)
Durante a trilha, o guia avisou o grupo para ter cuidado com qualquer inseto peçonhento escondido nas pedras.
(During the trail, the guide warned the group to be careful of any venomous insect hidden in the rocks.)
Uma aranha caranguejeira assusta pelo tamanho, mas outro aracnídeo menor pode ser muito mais peçonhento.
(A tarantula is scary because of its size, but a smaller arachnid can be much more venomous.)
O hospital da cidade recebeu um estoque novo de soro para tratar picadas de qualquer animal peçonhento comum na região.
(The city hospital received a new stock of antivenom to treat bites from any common venomous animal in the region.)
Summary
To easily remember the difference: if you touch or eat the organism and you suffer consequences, it is venenoso. However, if the organism bites or stings you to actively inject toxins, it is peçonhento. Furthermore, venenoso can describe a toxic personality, while peçonhento remains purely a scientific classification for armed venomous creatures.