put space between android buttons

<Button
    android:id="@+id/o_pharmacy"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:drawableLeft="@drawable/p2"
    android:text="@string/o_pharmacy"
    android:textSize="26sp" />

<Button
    android:id="@+id/lab"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:drawableLeft="@drawable/lab"
    android:text="@string/lab"
    android:textSize="26sp" />

<Button
    android:id="@+id/i_pharmacy"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:drawableLeft="@drawable/p1"
    android:text="@string/i_pharmacy"
    android:textSize="26sp" />

I tried above code to display 3 buttons in Liner layout. It works but I need to put space between the two buttons.