boost::iostreams::stream thread safety

Question

Can I write to boost::iostreams::stream from multiple threads without synchronization if my sink is thread-safe?

class my_sink : boost::iostreams::sink {
public:
    my_sink(param p) {}

    std::streamsize write(const char_type *s, std::streamsize n) {
        // thread-safe implementation
    }
};

boost::iostreams::stream<my_sink> my_stream(parameter);

void thread1() {
    my_stream << "some output";
}

void thread2() {
    my_stream << "some output";
}